[Build] Make sure `autogen.sh' can be run from anywhere.
[Build] Make sure `autogen.sh' can be run from anywhere.

When building in an external obj-dir this comes in handy.

jonnylamb/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx