In 2018 khronos changed the gl header guards. If we don't detect
this properly we would end up with plenty of symbol redifinition
(since we would be importing twice the "same" header).
Instead detect if the "newer" header was already included and if
so define the "old" define to avoid this situation
Fixes#523
Otherwise there may be no valid typedef of GLsync.
...
/usr/include/gstreamer-1.0/gst/gl/gstglfuncs.h:93:24: note: in definition of macro 'GST_GL_EXT_FUNCTION'
ret (GSTGLAPI *name) args;
^~~~
/usr/include/gstreamer-1.0/gst/gl/glprototypes/sync.h:33:23: error: 'GLsync' has not been declared
(GLsync sync))
^~~~~~
...
https://bugzilla.gnome.org/show_bug.cgi?id=796879
