llvm-flang compilation

I have just completed the build of llvm-flang.
The procedure was almost the same as the official;
https://github.com/flang-compiler/flang

However, my environment is RH6.4, somehow out-dated, and needs a little effort.
(1) Need to install a newer GCC, probably, 4.7 or newer (libstdc++4.7 is required).
    My case, I used 7.1
(2) at the cmake phase "-DGCC_INSTALL_PREFIX=/opt/GCC/7.1.0" is needed.
   Otherwise, clang/clang++ complains there is no proper libstdc++.