GCC 6.x remove -isystem include for system paths
This MR is a fix for GCC 6.x to only include FFTW path if not a system path.
It also ensures FFTW variables only defined if FFTW is required and to ensure the header is correctly found, rather than assuming it is in an include directory at the same level as the library directory (not true with multi-arch).
Closes #48 (closed) .