Edward Hervey authored Dec 17, 2018 and Tim-Philipp Müller committed May 16, 2019

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