BUG : clean fftw threads only if fftw is used
Bug description : MR !304 (merged) fixed a bug with fftw, at the end of the Mellin test we need to clean fftw threads by calling
But this is only true if fftw is actually used, i.e. if ITK was built using fftw (using cmake build option
ITK_USE_FFTWD) which is not mandatory for performing Fourier transforms with ITK, and the method
fftw_cleanup_threads(); is not avalaible without fftw.
This means that, with fix !304 (merged), OTB does not compile if ITK was built without fftw. This second fix adds a test on
ITK_USE_FFTWD before calling