gl: Allow using non-system mesa with both GL and GLES
GCC automatically disable redundance warnings for system headers. As soon as we start using a non-system installed mesa, we would start having issues. The test for both wasn't setting any flags, so it would work but then fail at runtime. This is being fixed by disabling in the code (where needed only) that GCC warning. The test is also fixed to avoid the false positive we had.
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