* actually everything is updated to their current HEADs but the aforementioned versions are close.
* Scintilla uses threads now, so we added checks for pthread.
  To be on the safe side, we imported AX_PTHREAD from the Autoconf archives.
  The flags are kept out of the ordinary build system, though and used only for compiling Scintilla
  and for linking.
  SciTECO may also use threads, but via Glib.
* Scinterm removed SCI_COLOR_PAIR(), so we re-added it to src/interface-curses/interface.c.
* There is an Asciidoc lexer now.
* The <Ix$> interruption bug (see TODO) is not fixed by this upgrade.
  Perhaps the Mac OS version runs better now. Feedback is needed (refs #12).

* This simplifies writing CFLAGS="-g -O0" CXXFLAGS="-g -O0".
* We build "release" binaries by default.
  NDEBUG will now be defined unless you specify --enable-debug.
  This enables some optimizations that have long been implemented but were never actually active:
  * SciTECO shuts down faster since it will not explicitly free memory.
    On the downside, this would complicate memory debugging with Valgrind/memcheck.
  * dlmalloc is built with -DINSECURE=1 which is supposedly a bit faster.
    Some compilers also complained about an unportable preprocessor usage which should
    now be gone.
* All CI builds are now with --enable-debug.
  This will slow them down but ensure that more code is executed and thus tested.

* Fixes warnings when checking for C++ 17 during ./configure
* Upstream source and documentation:
  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html

* Using modern GObject idioms and macros greatly reduces the necessary boilerplate code.
* The plain C versions of our GObject classes are now "final" (cannot be derived)
  This means we can hide the instance structures from the headers and avoid using
  explicit private fields.
* Avoids some deprecation warnings when building the Gtk UI.
* GOB2 is apparently no longer maintained, so this seems like a good idea in the long run.
* The most important reason however is that there is no precompiled GOB2 for Windows
  which prevents compilation on native Windows hosts, eg. during nightly builds.
  This is even more important as Gtk+3 is distributed on Windows practically
  exclusively via MSYS.
  (ArchLinux contains MinGW gtk3 packages as well, so cross-compiling from ArchLinux
  would have been an alternative.)

This is a total conversion of SciTECO to plain C (GNU C11).
The chance was taken to improve a lot of internal datastructures,
fix fundamental bugs and lay the foundations of future features.
The GTK user interface is now in an useable state!
All changes have been squashed together.

The language itself has almost not changed at all, except for:

* Detection of string terminators (usually Escape) now takes
  the string building characters into account.
  A string is only terminated outside of string building characters.
  In other words, you can now for instance write
  I^EQ[Hello$world]$
  This removes one of the last bits of shellisms which is out of
  place in SciTECO where no tokenization/lexing is performed.
  Consequently, the current termination character can also be
  escaped using ^Q/^R.
  This is used by auto completions to make sure that strings
  are inserted verbatim and without unwanted sideeffects.
* All strings can now safely contain null-characters
  (see also: 8-bit cleanliness).
  The null-character itself (^@) is not (yet) a valid SciTECO
  command, though.

An incomplete list of changes:

* We got rid of the BSD headers for RB trees and lists/queues.
  The problem with them was that they used a form of metaprogramming
  only to gain a bit of type safety. It also resulted in less
  readble code. This was a C++ desease.
  The new code avoids metaprogramming only to gain type safety.
  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle
  (https://github.com/jstimpfle/rb3ptr).
  This implementation is also more memory efficient than BSD's.
  The BSD list.h and queue.h has been replaced with a custom
  src/list.h.
* Fixed crashes, performance issues and compatibility issues with
  the Gtk 3 User Interface.
  It is now more or less ready for general use.
  The GDK lock is no longer used to avoid using deprecated functions.
  On the downside, the new implementation (driving the Gtk event loop
  stepwise) is even slower than the old one.
  A few glitches remain (see TODO), but it is hoped that they will
  be resolved by the Scintilla update which will be performed soon.
* A lot of program units have been split up, so they are shorter
  and easier to maintain: core-commands.c, qreg-commands.c,
  goto-commands.c, file-utils.h.
* Parser states are simply structs of callbacks now.
  They still use a kind of polymorphy using a preprocessor trick.
  TECO_DEFINE_STATE() takes an initializer list that will be
  merged with the default list of field initializers.
  To "subclass" states, you can simply define new macros that add
  initializers to existing macros.
* Parsers no longer have a "transitions" table but the input_cb()
  may use switch-case statements.
  There are also teco_machine_main_transition_t now which can
  be used to implement simple transitions. Additionally, you
  can specify functions to execute during transitions.
  This largely avoids long switch-case-statements.
* Parsers are embeddable/reusable now, at least in parse-only mode.
  This does not currently bring any advantages but may later
  be used to write a Scintilla lexer for TECO syntax highlighting.
  Once parsers are fully embeddable, it will also be possible
  to run TECO macros in a kind of coroutine which would allow
  them to process string arguments in real time.
* undo.[ch] still uses metaprogramming extensively but via
  the C preprocessor of course. On the downside, most undo
  token generators must be initiated explicitly (theoretically
  we could have used embedded functions / trampolines to
  instantiate automatically but this has turned out to be
  dangereous).
  There is a TECO_DEFINE_UNDO_CALL() to generate closures for
  arbitrary functions now (ie. to call an arbitrary function
  at undo-time). This simplified a lot of code and is much
  shorter than manually pushing undo tokens in many cases.
* Instead of the ridiculous C++ Curiously Recurring Template
  Pattern to achieve static polymorphy for user interface
  implementations, we now simply declare all functions to
  implement in interface.h and link in the implementations.
  This is possible since we no longer hace to define
  interface subclasses (all state is static variables in
  the interface's *.c files).
* Headers are now significantly shorter than in C++ since
  we can often hide more of our "class" implementations.
* Memory counting is based on dlmalloc for most platforms now.
  Unfortunately, there is no malloc implementation that
  provides an efficient constant-time memory counter that
  is guaranteed to decrease when freeing memory.
  But since we use a defined malloc implementation now,
  malloc_usable_size() can be used safely for tracking memory use.
  malloc() replacement is very tricky on Windows, so we
  use a poll thread on Windows. This can also be enabled
  on other supported platforms using --disable-malloc-replacement.
  All in all, I'm still not pleased with the state of memory
  limiting. It is a mess.
* Error handling uses GError now. This has the advantage that
  the GError codes can be reused once we support error catching
  in the SciTECO language.
* Added a few more test suite cases.
* Haiku is no longer supported as builds are instable and
  I did not manage to debug them - quite possibly Haiku bugs
  were responsible.
* Glib v2.44 or later are now required.
  The GTK UI requires Gtk+ v3.12 or later now.
  The GtkFlowBox fallback and sciteco-wrapper workaround are
  no longer required.
* We now extensively use the GCC/Clang-specific g_auto
  feature (automatic deallocations when leaving the current
  code block).
* Updated copyright to 2021.
  SciTECO has been in continuous development, even though there
  have been no commits since 2018.
* Since these changes are so significant, the target release has
  been set to v2.0.
  It is planned that beginning with v3.0, the language will be
  kept stable.

 * especially to improve building on FreeBSD 11
 * We need GNU Make, yet alone because Scintilla/Scinterm
   needs it. We now document that dependency and added
   an Autoconf check from the autoconf-archive.
   We make sure that the build process is invoked with GNU make
   by generating only GNUmakefiles.
   The Makefile.am files have not been renamed, so this
   change can be rolled back easily.
 * Some GNU-Make-specific autoreconf warnings have still been
   resolved. But not all of them, as this would have been
   unelegant and we need GNU Make anyway.
 * Declare ACLOCAL_AMFLAGS to appease autoreconf
 * Added an explicit check for C++11 from the autoconf-archives.
   In general we should support building with every C++11 compiler
   that is sufficiently GNU-like.
 * Do not use `sed` for inplace editing, as different sed-implementations
   have mutually incompatible syntax for this.
   Instead of declaring and checking a dependency on GNU sed,
   we simply use SciTECO for the editing task.
   This improves code portability on BSDs.
 * Similarily, BSD/POSIX `cmp` is supported now.
   This fixes the test suite on BSD without declaring a
   dependency on the GNU coreutils.
 * Simplified sciteco-wrapper generation.

 * Allows us to check for the Gob2 version at ./configure time
 * this file ships with Gob2 installations, so in most cases
   it could be found without shipping it with SciTECO.
 * Autoconf is built such that source distributions will contain
   all additional external macros compiled in aclocal.m4.
 * However if somebody builds from Git, he/she would still
   expect the ./configure checks to produce meaningful results
   even if not all dependencies are installed properly.
   It therefore seems to be good practice to include all
   external M4 macros (gob2.m4) as a fallback with the source tree.
 * /usr/share/aclocal contains many more useful m4 macros.
   However since we can depend on pkg-config e.g. for finding
   Gtk+ and Glib, I won't use those macros as else I would
   have to bundle them to achieve the same kind of ./configure
   robustness.