Turns out that the FreeBSD ports Makefiles install programs and scripts with 0555 by default,
i.e. without write permissions.
This broke the install-exec-hook unless building as root.
Unfortunately, this also broke the Poudriere builds at the FreeBSD build servers.
See https://pkg-status.freebsd.org/package19/data/141amd64-default-build-as-user/aa1f9b124e36/logs/errors/sciteco-curses-2.2.0.log

* They are used at runtime only by the GTK port.
* Their existence can cause problems if OS-specific build systems
  have to clean these files from the staging directory afterwards.
  This was the case on FreeBSD where the committer refused to remove
  these files after installation.
  In the official FreeBSD port, we therefore currently ship the
  PNG icons unnecessarily.
* They are now installed and shipped only on GTK builds.

This is not done automatically when building ports.
The FreeBSD binaries were thus the only ones without LTO.
We should prefer WITH_LTO, even once we add something like --enable-lto,
as WITH_LTO enables Clang-specific ThinLTO support.

Rodrigo Osorio re-included the PNGs even for sciteco-curses.
Should be fixed at the Autoconf-level, by only installing the PNGs on GTK.

* this works by embedding the SciTECO parser and driving it always (exclusively)
  in parse-only mode.
* A new teco_state_t::style determines the Scintilla style for any character
  accepted in the given state.
* Therefore, the SciTECO lexer is always 100% exact and corresponds to the current
  SciTECO grammer - it does not have to be maintained separately.
  There are a few exceptions and tweaks, though.
* The contents of curly-brace escapes (`@^Uq{...}`) are rendered as ordinary
  code using a separate parser instance.
  This can be disabled with the lexer.sciteco.macrodef property.
  Unfortunately, SciTECO does not currently allow setting lexer properties (FIXME).
* Labels and comments are currently styled the same.
  This could change in the future once we introduce real comments.
* Lexers are usually implemented in C++, but I did not want to draw in C++.
  Especially not since we'd have to include parser.h and other SciTECO headers,
  that really do not want to keep C++-compatible.
  Instead, the lexer is implemented "in the container".
  @ES/SCI_SETILEXER/sciteco/ is internally translated to SCI_SETILEXER(NULL)
  and we get Scintilla notifications when styling the view becomes necessary.
  This is then centrally forwarded to the teco_lexer_style() which
  uses the ordinary teco_view_ssm() API for styling.
* Once the command line becomes a Scintilla view even on Curses,
  we can enabled syntax highlighting of the command line macro.