Detect OpenMP-based thread parallel FFTW at configure time

If a system uses an FFTW library that was compiled using OpenMP parallelization rather than pthread, we do not detect that and fail to see that the library is actually installed since it needs linking with -lfftw3_omp -lfftw3 rather than -lfftw3_threads -lfftw3. Also, we then need to additionally pass -fopenmp (or whatever compiler equivalent)

On systems where FFTW is only installed with OpenMP support and without a non-threaded version altogether our configuration then fails. This is solved by passing the correct LDFLAGS by hand but is not ideal.