Import OpenAL Soft 1.15.1.

We're using OpenAL Soft in the Win32 build to eliminate the need for the
OpenAL SDK (which is no longer officially available).

Note that the headers are at the top level of the "include" directory.
This is to match how the OpenAL 1.1 SDK was set up on Windows.

Closes #232.
Remove SDL_mixer libs from Win32 build.

Now that we have OpenAL Soft, we don't have a need for SDL_mixer in the
Win32 build.
Merge remote-tracking branch 'origin/master' into cmake