It was found out in the course of NixOS#102078 that the libretro.mame*
packages did not build on macOS, both because of an unconditional
dependency on alsa-lib (which is exclusive to Linux) and references
to GCC scattered across makefiles.

This commit makes alsa-lib a dependency only on Linux, and tries to
force usage of the generic `cc`/`c++` commands.  Ideally we'd refer
to the `$CC` and `$CXX` environment variables, but that seems to
introduce recursive expansion problems for make.

Thanks to @SuperSandro2000 for suggestions and actual testing.

Wayland is not used or supported in other platforms.