build: move syntax-check code into build-aux directory

build: move syntax-check code into build-aux directory

The syntax-check rules are the one bit of make usage that will
stay around for a while after the meson conversion. Move them
into the build-aux directory in preparation for refactoring
to make them independent from automake.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>