Use SDL_main.h to rename main() to SDL_main().

Multiple platforms require SDL_main(), and providing main()
instead can cause various problems. This seems to be the best way
to rename main() and ensure C linkage.
Support compiling without CD player.

This adds preprocessor conditionals around CD player code, and
autoconf.ac logic for enabling that code. CD player icons are
still always included in the executable, wasting space.
This is meant to allow quicker porting and plugin development.