Modify the m4 file for fft to allow linking against MKL FFTs without giving...

Description

If one uses MKL, the FFTs used are the one of MKL, irrespective of specifying an FFT library, e.g. FFTW. This merge request modifies the m4 file for fft to allow linking against MKL FFTs without giving FFTW, which is not used anyway.

With the modified file, in case of using MKL, it is not necessary to specify the option "--with-fftw-prefix", which is simply replaced by export FCFLAGS_FFTW="-I$MKL_HOME/include/fftw"

News snippet

Modify the m4 file for fft to allow linking against MKL FFTs without giving FFTW, which is not used anyway.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.