Add check for MKL for build

[litearc]

Hi, I'm adding xtensor-fftw to our build process at work. We don't use FFTW, but use a compatible implementation through MKL. The CMakeLists file however doesn't check for this. Could you please add a check for MKL? Thanks!

[wolfv]

Hi, would you be interested in opening a PR for that? Would be a great help!

[egpbos]

Indeed sounds very useful! A PR in this case should go to the FindFFTW cmake package found here: https://github.com/egpbos/findfftw. FindFFTW is used in xtensor-fftw as a submodule.

[litearc]

Sure, I can work on it.