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.
This array is only used by the malloc failure test, and
defining it unconditionally creates an unused variable
warning on platforms where MALLOC_FAIL isn't set.
This test only works on glibc, but the important thing is support
for the __malloc_hook api, so that's what we should depend on,
especially since not all glibc versions have it.
The glibc 2.14 NEWS file says __malloc_hook will be removed
in the next release, so future-proof our use by checking
for this symbol at configure time and only compiling the
malloc failure tests if it is present.
