configure.ac: when checking for gl2ext.h include gl2.h
configure.ac: when checking for gl2ext.h include gl2.h

This makes sure we include gl2.h when looking for gl2ext.h since
gl2ext.h can't always be used standalone.

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