[build] Spring cleanup of configure.ac
[build] Spring cleanup of configure.ac

Let's try to bring configure.ac into this century, shall we?

* Use the proper shell macros AS_IF and AS_CASE instead of if...fi
  and case...esac

* Check for X11 and relative extensions only when building for GLX
  and EGLX backends

* Add documentation on the behaviour of binary_age and interface_age

* Coalesce all the common checks to avoid redundancy

* Escape everything that requires escaping

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