For the OpenCL version of the Resampling FStat we want to link this OpenCL FFT library together with the clFFT (https://github.com/clMathLibraries/clFFT , see https://gitlab.aei.uni-hannover.de/einsteinathome/lalsuite/issues/5). This PR therefore renames the library and headers to 'eclfft' (E@H's OpenCL FFT) to avoid naming conflcts with the clFFT. Note: this will require to change all code that uses this library, but that should only concern the BRP search code.