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.

These pragmas were adding in 4.6, and earlier versions warn
about them being undefined.

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.

This just works around the deprecation in glibc 2.14. I
do worry what will happen if they follow through and
remove it in 2.15.

Let's put it back in if/when we have a clearer view of voice/music detection

Renames test_opus to opus_demo and adds the test_opus_api, test_opus_encode, test_opus_decode test programs.