Merge branch 'cmake_fft' into 'master'

Fix CMake conditional include of the FFT module

Closes #191

See merge request walberla/walberla!601

src/CMakeLists.txt

@@ -29,7 +29,7 @@ if ( CMAKE_CUDA_COMPILER )
 endif()
 add_subdirectory( domain_decomposition )
 add_subdirectory( executiontree )
-if ( FFTW3_FOUND )
+if ( WALBERLA_BUILD_WITH_FFT AND FFTW3_FOUND )
    add_subdirectory( fft )
 endif()
 add_subdirectory( field )