Use AC_SEARCH_LIBS to search for a library containing math functions

Even though the previous mechanism worked, this should be more
correct.
Support streamed wav files with a 0 bytes data chunk
Allow opening stdin when "-" is specified
Allow reading wav files with a 0 bytes RIFF chunk

This is required in order to read streamed wav files produced by
the libavformat wav muxer.
Use skip instead of fseek

fseek(SEEK_CUR) doesn't work for nonseekable streams (such
as pipes).

Only do this for skipping past small chunks; don't use it for
skipping past the actual data for non-streamed input.
Support reading sox style piped wav files

Sox writes the RIFF header size as a very large number, instead
of as 0 as in libavformat.
Add support for waveformatex files

This is used for multichannel wave files.