We can't just rely on the `buildtype` option to decide which flags to
add, as the user can directly set the `debug` and `optimization`
options, which usually result in a `buildtype` option set to `custom`.
It also seems that `buildtype=debugoptimized` corresponds to a
production build, for which we shouldn't enable debug logs (see
reference below).

This doesn't exactly reproduce the autotools build mode of the
`--enable-debug` option, which we should probably accept having to
deviate from:
* `--enable-debug={full,yes}` are mixed up in the same debug mode, where
  all debug logs are enabled (which is perhaps no bad thing?). This
  corresponds to `buildtype=debug`.
* `--enable-debug=minimal` corresponds to `buildtype=debugoptimized`,
  where no flag is set.
* `--enable-debug=no` corresponds to `buildtype={minsize,release}`,
  where we set aggressive optimization flags.
* `buildtype=plain` is also a production mode, for which we don't set
  debug flags, but for which we don't really know the optimization mode,
  so we don't set aggressive optimization flags either.

Related: !98
See-also:
https://blogs.gnome.org/mcatanzaro/2022/07/15/best-practices-for-build-options/
https://mesonbuild.com/Builtin-options.html

This adds more detailed information about gobject-introspection, as well
as how to build vala API descriptions.  It also includes information
about detecting the presence of library functions, libm, and shipping
the meson build files in the autotools tarball.

Adds VERSION_FULL and PACKAGE_VERSION_FULL to xfce-revision.h, which can
be more easily used in apps.

Fixes version component splitting; previously it split on 'git', but it
should be '-dev' now.

Adds a 'version_short' meson.build variable for convenience.