build: Improve installation on system paths

Instead of being hardcoded, D-Bus, systemd and GIO modules paths
can be checked by using the information in their correspondant
pkg-config files.

This patch uses the information on pkg-config files by default,
and also allows the user to provide this information to avoid
overwriting system files.

https://bugzilla.gnome.org/show_bug.cgi?id=786149