* 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

 * 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.