also build fftw3_threads libraries, and enhance sanity checks
clarify comment w.r.t. no quad precision in Intel-based FFTW builds
Merge branch 'develop' into fftw_threads
fix remark, sync new FFTW 3.3.4 easyconfigs
fix sanity check for FFTW v3.3.4 with Intel-based toolchains (no quad)
Merge pull request #1013 from boegel/fftw_threads

also build fftw3_threads libraries, and enhance sanity checks