Re-enable MKL support for FFTs

[tskisner]

Our internal FFT routines have had support for directly using MKL for years. However, this was disabled when we moved to our modern CMake build system. This PR adds checks for MKL and if found does not look for FFTW. This works around the frequent issue where MKL ships with a partial FFTW interface and so linking to both libraries can break our code.

Tested on cori.nersc.gov with Intel compilers and MKL.

[tskisner]

Although statistical unit tests pass with both FFT frameworks, I am working on comparing generated timestream values to make sure that they are the same. However, the build system changes in this PR can be reviewed already.

[tskisner]

For the record, I ran a comparison using the toast_satellite_sim.py example script and the following parameters:

--group-size
4
--nside
256
--no-day-maps
--no-season-maps
--no-sky
--simulate-noise
--sample-rate
20.0
--obs-time-h
1.0
--obs-num
10
--hwp-rpm
5.0
--madam
--madam-iter-max
10
--madam-baseline-length
60.0
--no-destripe
--no-wcov
--no-wcov-inv
--binmap
--hits
--focalplane
../fp_7.pkl

With a synthetic focalplane made with

toast_fake_focalplane.py \
    --minpix 7 \
    --out fp \
    --fwhm 10.0 \
    --fov 10.0 \
    --psd_fknee 0.1 \
    --psd_alpha 1.0 \
    --psd_fmin 1.0e-5 \
    --psd_NET 1.0 \
    --bandcenter_ghz 150.0 \
    --bandwidth_ghz 40.0

I used one cori KNL node with 16 processes. I ran this script with the cmbenv-gcc/20200630 and cmbenv-intel/20200630 software stacks (which have toast 2.3.8) and verified that the outputs agreed. Next I took the cmbenv-intel stack and compiled this branch of toast using just MKL without any linking to FFTW. However, note that the pipeline above does have other python packages that link to FFTW (Madam). This compilation of toast passes all unit tests and the outputs of the above pipeline agree with the other two stacks.