Fix exit status of gen-resources.py
Fix exit status of gen-resources.py

If glib-compile-resources did not produce anything on stdout,
gen-resources.py reports success, even if it actually failed. This
can lead to a build error down the line.

Instead, just use os.execvpe after setting up the environment.

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf