qtgl: Handle OPENGL header guard changes

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
6 jobs for qtglfix in 67 minutes and 7 seconds (queued for 1 minute and 49 seconds)
Status Job ID Name Coverage
  Preparation
passed #65074
gst indent

00:00:24

passed #65073
manifest

00:00:19

 
  Build
passed #65075
build fedora x86_64

00:03:00

 
  Test
passed #65076
check fedora

00:16:26

 
  Full Builds
passed #65077
build android arm64 api28

00:02:22

passed #65078
build cerbero fedora x86_64

00:44:32