src/caf.c : Accept non-audio chunks after 'data' chunk.

Closes : https://github.com/erikd/libsndfile/issues/50
Detect Clang compiler compiler.

Autoconf detects the Clang compiler as GNU GCC (clang sets defines like
__GNUC__ etc) but Clang is *not* completely compatible. We detect Clang
and set ac_vc_c_compiler_gnu to 'no'.

Closes: https://github.com/erikd/libsndfile/issues/49