cmake: Check for includes after testing the enviroment

Change-Id: If0819f75cc85f23f904dd72c69aa7631aeba2ec9
cmake: Add -Wno-gnu

Pass the '-Wno-gnu' option to compilers that support it.

This prevents clang from complaining about certain GNU extensions used
in LizardFS.

Change-Id: I51554c1bc31b7134cc2299da076570cc72824b05