libao: add version info to ao/ao.h

As new features are added to libao, version information in the ao.h header would be helpful so code that uses libao can detect the version and use the new features.

For example, the ao_sample_format has a new field called matrix in 1.x.x that is not int version 0.8.8 (the version currently in the Ubuntu repositories 10.04, 11.04).

I'm releasing code that will work with 0.8.8 but it would be nice to be able to include code for 1.1.0 when it becomes available in the Ubuntu repositories.