Support for icc and ifort.
It is possible to use MKL's FFT through their FFTW wrappers
Clean up compiler warnings
Use rem_pio2 from openspecfun. Allows other libms to be used again.
Fix #5365
Rename USE_MKL -> USE_INTEL_MKL and add deprecation
Introduce USE_INTEL_MKL_FFT
Disable dsp and fft tests that are not supported by MKL's fftw wrappers