What's the best way to have an ebuild abort if two mutually exclusive
USE flags are both set?
I'm trying the modify the mozilla-firefox and xulrunner ebuilds to
support building with OSS instead of ALSA (sadly, the current versions
of the ebuilds in Portage only allow ALSA to be enabled, not OSS), but
the build system of Xulrunner/Firefox does not allow to build with both.
So I need to abort when both "alsa" and "oss" are set rather than having
the build proceed and the user getting a compilation error later on.