fade: Infrastructure for the modular mixer API.

This commit prepares for supporting more than one mixer by introducing
$mixers in configure.ac which contains the set of supported mixers. It
creates an empty oss_mix.c file to which the oss-specific code will
be moved in the next patch.

The fade-specific stuff of configure.ac is moved to a separate section
since whether to build fade will soon no longer depend on OSS only.

The patch is a bit large due to some generic m4 macros being moved to
the top of configure.ac as they are needed in the new fade section.