gl: provide platform specific pkg-config files
Follow-up from #735 (closed).
I think, [we want to provide] at least one [
gstreamer-gl-*pkg-config file] for each of the extra platform-specific headers. At the moment that is x11, wayland, egl and gl-prototypes. Others may be useful instead of the pc variables in
GST_GL_HAVE_*from gstglconfig.h) but I don't think adding one for each option is strictly required. This also would conveniently match nicely with the extra .gir files needed for platform-specific functions.