xorg-launch-helper: use virtual/pkg-config

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
Fix up compton[scm], devel and master branch merge

The developer of compton has merged his dev branch into master.
master branch is now to be considered the proper branch for testing.

Currently this fails when dbus 1.7 is installed complaining about
 * "libdbus-1.so: undefined reference to `mq_getattr'"

Google suggest that it could be related to glibc 2.[16,17] and dbus 1.7