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.