Error in detecting CMake in later versions for FFTW
From Björn Dick at HLRS:
a hint regarding cmake/ThirdPartyFFTW.cmake:63 in branch feature/IncNS_Sensor: CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES seems to be not set anymore in recent versions of cmake (at least 3.5.2). So my attempt to build failed due to unallowed number of arguments (CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES is an empty string here). It set it by hand now, but maybe we should fix this in the repo.