Automagically generate an AndroidManifest with the right Crawl version.
Update .gitignore with more Android stuff.
Actually build against contrib's SDL2 headers.

How this built before, I have no idea.
Bump sdl2-image contrib, allowing x86 Android target again.

Hooray.
Gut and largely rewrite the Android build documentation.
Re-enable old Android port's sound with an SDL2_mixer contrib.

Yay.
Use SDL2_Mixer to play sounds on non-Android platforms too.

Disabled if we're using the old Windows sound support.
Touch INSTALL.txt to indicate the new dependencies and sound support.
Touch SDL2_mixer contrib for OS X build.