The attempted consistency check to remove FLTK's obsolete cache
variable OPENGL_GLU_INCLUDE_DIR which is used by CMake 3.29+ could
cause false positives, at least on macOS, as reported on issue #1046.
This modified code removes OPENGL_GLU_INCLUDE_DIR if it is defined
but FLTK_OPENGL_GLU_INCLUDE_DIR is not (yet) defined which indicates
the first configure execution with the new code.