configure.ac: Use pkg-config for gles1/2 if available
configure.ac: Use pkg-config for gles1/2 if available

When building for gles1/2 we now always try and look for a libglesv1_cm
or libglesv2 .pc file with details about where to find the library and
headers.

nobled/clutter,kerrickstaley/clutter-vala,djdeath/clutter,rkudiyarov/clutter_osx,spatulasnout/clutter,collects/clutter,djdeath/clutter-multithreaded,djdeath/clutter-multithreaded,jigpu/clutter,djdeath/clutter,GNOME/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter-android,GNOME/clutter,collects/clutter,jigpu/clutter,GNOME/clutter,heysion/clutter-clone,nobled/clutter,djdeath/clutter,heysion/clutter-clone,GNOME/clutter,djdeath/clutter,djdeath/clutter-android,heysion/clutter-clone,ebassi/clutter,kerrickstaley/clutter-vala,collects/clutter,GNOME/clutter,jigpu/clutter,spatulasnout/clutter,heysion/clutter-clone,Distrotech/clutter,ebassi/clutter,kerrickstaley/clutter-vala,djdeath/clutter-android,Distrotech/clutter,jigpu/clutter,kerrickstaley/clutter-vala,heysion/clutter-clone,Distrotech/clutter,rkudiyarov/clutter_osx,collects/clutter,nobled/clutter,jigpu/clutter,djdeath/clutter,dlespiau/clutter,djdeath/clutter-multithreaded,rkudiyarov/clutter_osx,spatulasnout/clutter,dlespiau/clutter,ebassi/clutter,djdeath/clutter-multithreaded,ebassi/clutter,spatulasnout/clutter,Distrotech/clutter,collects/clutter,dlespiau/clutter,Distrotech/clutter,djdeath/clutter-android,rkudiyarov/clutter_osx,Distrotech/clutter,ebassi/clutter,spatulasnout/clutter,djdeath/clutter-android