Revert "No need to have OpenCL optional at configure-time"

As long as this requires bundling clcc, not allowing to disable it is
unacceptable.

This reverts commit 3b0b76462aa54f60bc9823d9c6ef64e5e35c1208.

Conflicts:
	configure.ac

Change-Id: I12b3d5e3325bbd03cd98e1fb5139705d694493c2