build: Check for gdk-pixbuf
build: Check for gdk-pixbuf

Some tests may need to load image data without going through
ClutterTexture or Cogl, so let's add a configure switch and a
conditional for gdk-pibxuf.

Distrotech/clutter,jigpu/clutter,jigpu/clutter,collects/clutter,GNOME/clutter,ebassi/clutter,GNOME/clutter,kerrickstaley/clutter-vala,jigpu/clutter,djdeath/clutter-android,ebassi/clutter,djdeath/clutter,jigpu/clutter,ebassi/clutter,GNOME/clutter,Distrotech/clutter,djdeath/clutter-android,Distrotech/clutter,collects/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter-multithreaded,heysion/clutter-clone,djdeath/clutter,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,Distrotech/clutter,GNOME/clutter,heysion/clutter-clone,ebassi/clutter,Distrotech/clutter,djdeath/clutter-android,heysion/clutter-clone,heysion/clutter-clone,collects/clutter,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,djdeath/clutter-multithreaded,djdeath/clutter,Distrotech/clutter,heysion/clutter-clone,djdeath/clutter,collects/clutter,GNOME/clutter,djdeath/clutter-android,kerrickstaley/clutter-vala,jigpu/clutter,collects/clutter,djdeath/clutter,djdeath/clutter-android,ebassi/clutter