configure.ac : Add detection of -fsanitizer=undefined.
configure.ac : Add detection of -fsanitizer=undefined.

Both Clang and GCC-4.9 now support the undefined behaviour sanitizer
so detect and enable it when configuring with --enable-sanitizer.

Also improve configure reporting of sanitizer and stack smash
protection.

syb0rg/libsndfile,erikd/libsndfile,RonNovy/libsndfile,erikd/libsndfile,syb0rg/libsndfile,libsndfile/libsndfile,RonNovy/libsndfile,erikd/libsndfile,evpobr/libsndfile,audiokit/libsndfile,evpobr/libsndfile,libsndfile/libsndfile,erikd/libsndfile,RonNovy/libsndfile,Icenowy/libsndfile,Icenowy/libsndfile,Icenowy/libsndfile,libsndfile/libsndfile,syb0rg/libsndfile,audiokit/libsndfile,evpobr/libsndfile,evpobr/libsndfile,erikd/libsndfile,libsndfile/libsndfile,libsndfile/libsndfile,Icenowy/libsndfile,RonNovy/libsndfile,audiokit/libsndfile,evpobr/libsndfile,syb0rg/libsndfile,Icenowy/libsndfile,audiokit/libsndfile