Added new FindclFFT script

Hi @kknox

This is an independent pull request for the FindclFFT.cmake script as discussed in [your last comment in pull request 33](https://github.com/clMathLibraries/clFFT/pull/33#issuecomment-34506048).

I noticed that the version of clFFT [distributed on AMD's website](http://developer.amd.com/tools-and-sdks/heterogeneous-computing/amd-accelerated-parallel-processing-math-libraries/) installs the libraries to `/opt/clAmdFft-1.10.321`. Should we support this in the find script as well?