meson: Do less when not building from git

meson: Do less when not building from git

As explained in the comment, the syntax-check machinery uses git
to figure out the list of files it should operate on, so we can
only enable it when building from git.

Despite only registering the various tests with meson in that
case, however, we unconditionally perform a bunch of preparation
that is only useful for the purpose of registering and running
the tests. If we're not going to do that, we can skip a few steps
and save a bit of time.

Best viewed with 'git show -w'.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>