nixos opengl: add s2tc to mesa drivers by default

Close #2200. Thanks to @cpages for suggesting and testing this.
mesa: have all output on /run/opengl-driver{,-32}

Fixes #2242 in a different way (cleaner, I hope).
gsasl: fix build on Darwin (close #2260, fixes #2259)

@vcunat rewrote with "more standard" optional functions.