allow to override default OpenCL compile options by defining CLFFT_COMPILE_OPTIONS macro

In the radio pulsar search application, the one that this lib was originally developed for, we try to avoid fused multiple-add instructions, in particular when building the FFTW library. However, this library explicitly enables these ("-cl-mad-enable"). We need a way to override these (default) OpenCL compile flags externally to ensure compatibility (comparability of results).