ALAC/alac_encoder.c : Add correct byte swapping for mChannelLayoutTag.

Patch from Michael Pruett, author of libaudiofile.
M4/gcc_version.m4 : Fix to work with OpenBSD's sed.
tests/dft_cmp.c : Fix buffer overflow detected using gcc-4.8.

This was a buffer overflow in libsndfile's test suite code, not
in the actual library code.