added missing CMake FFTW variables to External_SIRF.cmake, fixes #197 (together with installing boost_1_65_1)
added missing CMake FFTW variables to External_SIRF.cmake, fixes #197 (together with installing boost_1_65_1)

CCPPETMR/SIRF-SuperBuild,CCPPETMR/SIRF-SuperBuild