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

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.

* 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