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 fftw_cleanup_threads();.

But this is only true if fftw is actually used, i.e. if ITK was built using fftw (using cmake build option ITK_USE_FFTWF or 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_FFTWF and ITK_USE_FFTWD before calling fftw_cleanup_threads().