* You no longer have to copy contrib/scintilla, contrib/scinterm and contrib/lexilla
  manually to the build directory.
* It turns out, that Scintilla/Lexilla was supporting this since 2016.
  Scintilla allows pointing to a source directory (srdir) and Lexilla to a binary directory (DIR_O).
* For Scinterm I opened a pull request in order to add srcdir/basedir variables:
  https://github.com/orbitalquark/scinterm/pull/21
* `make distcheck` is therefore now also fixed.
* The FreeBSD package is now allowed to build out of source.
  I haven't tested it yet.
* See also https://github.com/ScintillaOrg/lexilla/issues/266

* dh_testdir would sometimes cause problems on the PPA servers,
  so we replaced it with an empty rule.
* We need to define the binary-arch and binary-indep targets
  in order to recurse into the correct subdirectories.
  I do not understand why this always worked on the Github runner
  and sometimes even on the PPA servers.
  Unfortunately, we cannot just call `dh binary-arch -B... -p...`
  since that would call `dh_install -a` which overrides any `-p` (package),
  thus trying to install the curses binaries from the Gtk build directory
  and vice versa.

* fixed some Debian lintian warnings

* Previous Scintilla version was 3.6.4 and Scinterm was 1.7 (with lots of custom patches).
  All of the patches are now either irrelevant or have been merged upstream.
* Since Scintilla 5 requires C++17, this increases the minimum GCC version at least
  to 5.0. We may actually require even newer versions.
* I could not upgrade the scintilla-mirror (which was imported from Mercurial),
  so the old sciteco-dev branch was renamed to sciteco-dev-pre-v2.0.0,
  master was deleted and I reimported the entire Scintilla repo using
  git-remote-hg.
  This means that scintilla-mirror now contains two entirely separate trees.
  But it is still possible to clone old SciTECO repos.
* The strategy/workflow of maintaining hotfix branches on scintilla-mirror has been changed.
  Instead of having one sciteco-dev branch that is rebased onto new Scintilla upstream
  releases and tagging SciTECO releases in scintilla-mirror (to keep the commits referenced),
  we now create a branch for every Scintilla version we are based on (eg. sciteco-rel-5-1-3).
  This branch is never rebased or deleted. Therefore, we are guaranteed to be able to
  clone arbitrary SciTECO repo commits - not only releases.
  Releases no longer have to be tagged in scintilla-mirror.
  On the downside, fixup commits may accumulate in these new branches.
  They can only be squashed once a new branch for a new Scintilla release is created
  (e.g. by cherry-picking followed by rebase).
* Scinterm does no longer have to reside in the Scintilla subdirectory,
  so we added it as a regular submodule.
  There are no more recursive submodules.
  The Scinterm build system has not been improved at all, but we use
  a trick based on VPATH to build Scinterm in scintilla/bin/.
* Scinterm is now in Git and we reference the upstream repo for the
  time being.
  We might mirror it and apply the same branching workflow as with Scintilla
  if necessary.
  The scinterm-mirror repository still exists but has not been touched.
  We will also have to rewrite its master branch as it was a non-reproducible
  Mercurial import.
* Scinterm now also comes with patches for Scintilla which we simply applied
  on our sciteco-rel-5-1-3 branch.
* Scintilla 5 outsourced its lexers into the Lexilla project.
  We added it as yet another submodule.
* All submodules have been moved into contrib/.
* The Scintilla API for setting lexers has consequently changed.
  We now have to call SCI_SETILEXER(0, CreateLexer(name)).
  As I did not want to introduce a separate command for setting lexers,
  <ES> has been extended to allow setting lexers by name with the SCI_SETILEXER
  message which effectively replaces SCI_SETLEXERLANGUAGE.
* The lexer macros (SCLEX_...) no longer serve any purpose - they weren't used
  in the SciTECO standard library anyway - and have consequently been removed
  from symbols-scilexer.c.
  The style macros from SciLexer.h (SCE_...) are theoretically still useful - even
  though they are not used by our current color schemes - and have therefore been
  retained. They can be specified as wParam in <ES>.
* <ES> no longer allows symbolic constants for lParam.
  This never made any sense since all supported symbols were always wParam.
* Scinterm supports new native cursor modes.
  They are not used for the time being and the previous CARETSTYLE_BLOCK_AFTER
  caret style is configured by default.
  It makes no sense to enable native cursor modes now since the
  command line should have a native cursor but is not yet a Scintilla view.
* The Scintilla upgrade performed much worse than before,
  so some optimizations will be necessary.

* Added a Freedesktop file - only as part of the debian package yet.
  `make install` won't install a Desktop file since it would have to be
  generated.
* Just like when installing manually, you can have a Curses and Gtk installation
  side by side using the same .teco_ini.
* Common data between the Curses UI (sciteco-curses) and Gtk UI are in a new
  architecture-independant package sciteco-common.
* The Gtk+ binaries are prefixed with `g` (gsciteco, gtedoc.tes, ggrosciteco.tes).
* Debian source and binary packages can be built using `./distribute.mk debian`
  as usual.
  It should also be possible to push everything to the PPA for the next release,
  although that is not yet tested.

 * Usually, Scintilla will now be built with -O2
 * this can improve performance significantly over the standard Scintilla -Os
   (up to 10%).
 * this also allows link-time-optimizing both Scintilla and SciTECO
   (which are linked statically) by adding -flto to CFLAGS, CXXFLAGS and LDFLAGS.
   Link-time-optimization will both reduce the total binary size
   and improve performance slightly since scintilla_send_message() can be
   inlined.
   An -O3 optimized Scintilla when linked with LTO results in an only 300kb
   larger SciTECO binary.
 * the highest possible optimization thus requires the following maintainer
   flags on the ./configure command line:
   CFLAGS="-O3 -mtune=native -march=native -flto"
   CXXFLAGS="-O3 -mtune=native -march=native -flto"
   LDFLAGS="-flto"
 * Windows and Debian builds use link-time-optimization now.
   On Windows - where we link in everything statically - building
   the dependant libraries with -flto could improve performance
   even more.
 * Debian builds respect the default hardening flags of the build
   server now. This should ensure that SciTECO is built for the
   correct architecture at the recommended optimization level etc.

 * it also sets the compatibility level to 7 which was required
   when building for Ubuntu Lucid. This version cannot be supported
   however since its libglib version is too old.

this should simplify building SciTECO for new users

 * compiler and archiver are passed down from Autoconf,
   so cross-compiling should work transparently
 * `make clean` will also clean the Scintilla source tree
 * there is no longer any need for "source bundles" as
   tar balls also contain Scintilla/Scinterm now
 * building from Git is not much more difficult than building
   from a tar ball
 * The versions of Scintilla/Scinterm embedded as submodules
   already contain all the patches necessary (currently none are
   necessary), so there's no need to have patch files in the
   repository
 * INSTALL instructions have been rewritten
 * the --with-scintilla and --with-scinterm site-config options
   have been kept. But they should be rarely necessary now.

so no additional --with-scintilla is necessary when building from
a source bundle

 * prepared for upload to Ubuntu PPA
 * debian package will not be in autoconf source distro
 * will only build as a non-native package based on a source bundle
   that contains scintilla, scinterm and has patches applied