Just link that code into the sc library. We use clew anyway to handle
optoinal presence of OpenCL at run-time.

This is a straightforward change for now. Could be cleaner; we could
get rid of the fairly pointless wrappers in
sc/source/core/opencl/formulagroupcl.cxx for instance.

Change-Id: I23215c6516ef04698f6891e2a7fc6611dc83b37a