at-spi2: Make X11 support truly optional

X11 support in at-spi2-core can be turned off at compile time, so leverage this
and disable it when X11 is not present in DISTRO_FEATURES.

Signed-off-by: Philippe Normand <philn@igalia.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>