This allows us to do further probing in the included meson files:
Individual subdirectories (eg. DDXes, extensions, OS layer, ...)
can now probe things that are only relevant to them - no need to fill
the already too fat includes/meson.build with even more things.
Preparation for upcoming commits that'll make us of that.
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>