UPDATE:

    (*) Support older Darwin w/o clang++ by enabling g++ option.
    	However currently this assumes no C++11. Will modify if
	there's such case of using g++ & c++11 on Darwin.

    (*) Loosen the check on ROOTSYS: this is not needed to compile
    	LArLight (as Corey proved). Set warning if ROOTSYS is not
	set as PYTHONPATH won't be set automatically in such case.

  -Kazu