To get a certification you need to pay, and it's actually really expensive. That's why BSD-like and GNU/Linux OS vendors don't apply for it.
So there isn't even a reason to check whether GNU/Linux is compliant or not.
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Most of all, the GNU/Linux distribution follows the Linux Standard Base, which is free of charge, and recognized by almost all Linux vendors.
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**Edit:** As my answer is not completely correct, I'll add the @vonbrand comments:
> Linus (and people involved in the development of other parts of Linux distributions) follow the pragmatic guideline to make it as close to POSIX as is worthwhile. There are parts of POSIX (like the (in)famous STREAMS) that are ill-conceived, impossible to implement efficiently, or just codification of historic relics that should be replaced by something better.
... therefore, does it make it harder to obtain a certification?
> Sure. POSIX mandates some interface, which Linux just won't ever have. Case closed.