Make configure script work better under steamruntime

Steamruntime uses a gcc with a version much like the following:
i686-unknown-linux-gnu-gcc (crosstool-NG 1.17.0) 4.6.3
The new case matches that gcc (*-*-gcc\ *)

The steamruntime also has an sdl2-mixer with mp3 issues, but we want
to use it anyways. The --with-external-sdl-mixer skips all checks for
the mixer and uses the system one regardless