This adds the OSS audio backend and removes OSS code from
core/linux-dist/main.cpp and core/sdl/main.cpp.
The OSS backend will be included during compilation if the USE_OSS
flag is set.

Although OSS code in core/linux-dist/main.cpp depended on the
TARGET_PANDORA flag instead of USE_OSS, the latter should work too
since it is defined in the Pandora Makefile (lines 7 & 83).