havefftw: Check whether chebpol uses FFTW
Description
It is possible to compile chebpol without FFTW. If this is done, it
will not be feasible with high-degree Chebyshev polynomials. I.e. a
100x100x100 approximation will be possible, but not a one-dimensional 1000000.
This function checks whether chebpol uses FFTW.Value
- Returns TRUE if chebpol uses FFTW. Otherwise FALSE.