ThirdParty-plus FFTW version

The FFTW version that we have in our develop files (etc/config.sh/FFTW) has been withdrawn from the public domain. See from www.fftw.org/release-notes.html: FFTW 3.3.6-pl1 (withdrawn) Jan 16th, 2017 Bugfix: FFTW 3.3.6 had the wrong libtool version number, and generated shared libraries of the form libfftw3.so.2.6.6 instead of libfftw3.so.3.*.

We should probably move to -pl2 instead of –pl1 .