Seg3D Image Registrations tools build notes:

Windows:
- building FFTW as an ITK CMake external is NOT supported on Windows
- tried downloading precompiled library zip file from fftw website, 3.3.4 64-bit release
- enable ITK_USE_SYSTEM_FFTW
- set FFTW_INCLUDE_PATH=c:/fftw-3.3.4-dll64,
      FFTWD_LIB=c:/fftw-3.3.4-dll64/libfftw3-3.dll (not sure this is correct),
      FFTWF_LIB=c:/fftw-3.3.4-dll64/libfftw3f-3.dll
