Fix automake forward-incompatibility.
* Added VORBS check to autotools.
* Added -lm on linux as it seems necessary on arch.
* SDL2 should definately not be the default as it's pretty broken still it seems.
Check for sqrt(3), required for system csparse.
Use pkg-config to check for OpenGL and SDL_mixer.
Use default "not-found" behavior for pkg-config.

By default, pkg-config will fail with a detailed error message if a
package is not found; we do not need to specify our own handler.
* Don't actually need glu when compiling.