Add '--quiet' option to gnome.generate_gir() calls
Adding the --quiet argument to gnome.generate_gir() silences
informational build output such as what g-ir-scanner was called with.
Gtk and others do this, so why don't we as well?
<?xml version="1.0"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"