qtgl: Handle OPENGL header guard changes

Edward Hervey requested to merge bilboed/gst-plugins-good:qtglfix into master

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 (closed)

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