GLib guarantees libintl is always present, using proxy-libintl as last resort. There is no need to mock gettex API any more. This fix static build on Windows because G_INTL_STATIC_COMPILATION must be defined before including libintl.h, and glib does it for us as part as including glib.h. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2028>