meson: Use dependency().found() instead of conf.has()

meson: Use dependency().found() instead of conf.has()

So far this change alone doesn't make much sense, but prepares
code for upcoming change. Unfortunately, some conf.has()
statements have to stay, because there's no corresponding
dependency(). But that's okay.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>