Add missing FFI file.
Add missing prefixSum_kernels.cl.
Remove assumption about number of platforms in examples/PrefixSum.hs.

Should probably do something similar to the other examples.
Attempt to un-hard-code path to OpenCL include/lib directories.

As part of this, I had to add a Setup.hs file.  I have no idea if this
works on other platforms that support OpenCL; I think the new code I
added will only be triggered on Windows, but I can't test to make sure.
Merge pull request #1 from jgbm/master

Tidying