Fix the test for gcc version >= 4.6 to work for gcc 5 and up.

We use the __GNUC_PREREQ macro from celt/arch.h to check
the required minimum gcc version. The previous check would
have failed for gcc 5.n with n from 0 to 5.

Thanks to pcgod for pointing out the problem and derf for
pointing out we had a macro for this.