Integrate CLEW into osd library and examples

If the system has CLEW installed (which is detected by recently
added FindCLEW routines) then OpenSubduv would be compiled against
this library.

It makes binaries and libraries more portable across the systems,
so it's possible to run the same binary on systems with and without
OpenCL SDK installed.

The most annoying part of the change is updating examples to load
OpenCL libraries, but ideally code around controllers and interface
creation is to be de-duplicated anyway.

Based on the pull request #303 from Martijn Berger