gets rid of cmake FFTW warning

cmake/set_up_FFTW.cmake

@@ -50,5 +50,8 @@ else()
       PkgConfig::FFTW3F)
   endfunction()
 
-  return(PROPAGATE FFTW3_INCLUDE_DIRS)
+  get_directory_property(hasParent PARENT_DIRECTORY)
+  if (hasParent)
+    return(PROPAGATE FFTW3_INCLUDE_DIRS)
+  endif()
 endif()