The cocoa UI code currently assumes it is always the active UI
and does not interact well with other UI frontend code.  Move
the relevant checks to Meson now that all other frontends
have become Meson options.  This way, SDL/GTK+/Cocoa can be
parsed entirely by scripts/configure-parse-buildoptions.pl.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>