COMP: add FFTW library directory to FFT module
COMP: add FFTW library directory to FFT module

Fixes #1922.

vfonov/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,Kitware/ITK,richardbeare/ITK,vfonov/ITK,BRAINSia/ITK,thewtex/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,hjmjohnson/ITK,LucasGandel/ITK,hjmjohnson/ITK,richardbeare/ITK,hjmjohnson/ITK,hjmjohnson/ITK,BRAINSia/ITK,LucasGandel/ITK,hjmjohnson/ITK,vfonov/ITK,BRAINSia/ITK,Kitware/ITK,Kitware/ITK,Kitware/ITK,hjmjohnson/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,Kitware/ITK,richardbeare/ITK,vfonov/ITK,thewtex/ITK,BRAINSia/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,BRAINSia/ITK,thewtex/ITK,thewtex/ITK,thewtex/ITK,thewtex/ITK,vfonov/ITK,vfonov/ITK,richardbeare/ITK,hjmjohnson/ITK,richardbeare/ITK,vfonov/ITK,vfonov/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,LucasGandel/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK