Rewrite the port so we build our own Linux package from the upstream
.src.rpm with patches derived from the audio/alsa-plugins port. This
should improve sound output from Linux programs.
CFLAGS contains -nostdinc -isystem ... because gcc searches
/usr/local/include before /usr/include otherwise and picks up headers from
/usr/local/include/alsa instead of /compat/linux/usr/include/alsa.