This reverts commit 11829fe7d0.
The mkenums_simple function can't properly handle headers
in subdirectories currently, so go back to the template
version.
The use of volatile was incorrect in GLib and has been that way for
a long time. Recently however that has changed, and this makes GTK
follow suit to avoid using volatile in the type registration.
See also: https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719
Combined with the above merge request for GLib, this fixes a large
number of compilation warnings when using Clang.
The @filename@ directive will use the full path of the file being parsed
for enumeration types; we should use @basename@, instead, as it improves
the reproducibility of the build by using only the file name.
glib-mkenums is not currently clever enough to know which version an
enum type was added in, so just mark all the _get_type() functions as
available in all versions.
