Fix fftw_libs for MKL without interface libraries.

[bartoldeman]

PR #2082 allowed the use of MKL without interface libraries
(useful if neither FFTW2 nor MPI are needed in compiler+linalg
toolchains without MPI). However I forgot to adjust fftw_libs
and so the LIBFFT environment variable still included the
non-existing interface libraries.

This patch corrects this issue.

[boegel]

@bartoldeman We're doing the exact same thing to both check_fftw_libs and fftw_libs, so let's refactor things a bit and pull down the definition of check_fftw_libs to right above where it is used:

fftw_lib_exists = lambda x: any([os.path.exists(os.path.join(d, "lib%s.a" % x)) for d in fft_lib_dirs])
if not fftw_lib_exists(interface_lib) and LooseVersion(imklver) >= LooseVersion("10.2"):
    # interface libs can be optional:
    # ...
    fftw_libs = [l for l in fftw_libs if l not in [interface_lib, cluster_interface_lib]]

# filter out libraries from list of FFTW libraries to check for if they are not provided by Intel MKL
check_fftw_libs = [lib for lib in fftw_libs if lib not in ['dl', 'gfortran']]

if all([fftw_lib_exists(lib) for lib in check_fftw_libs]):
    self.FFT_LIB = fftw_libs
else:
    ...

[boegel]

lgtm, thanks @bartoldeman!