make sure ARM perf libs for FFTs / BLAS works
The ARM performance libraries implements FFTs with an FFTW3 compatible API; what's missing is the guru interface, but this is not a hard requirement in our FFT implementation.
For the 2019 release we should make sure that with ARMPL we don't use the guru interface API and that preferably back it with jenkins testing.
Add tweaks to support ARMPL for FFTs
Support added through the FFTW compatibilty layer; the FFTW guru
interface is not implemented in ARMPL, so this is disabled in builds
that use ARMPL. The detection is basic and it is used only for these
purposes, the lirbary and header location needs to be passed manually
(we prefer ARM to implement a Find module).