C++11 support.

* Add options and ENV method to specify building in C++11 mode.
  - Set C++ compiler flags to enable C++11 mode.
  - To add options to support C++11 mode, a formula can now use

      option :cxx11

    to provide "--cxx11" option, and detect and enable C++11 support in
    install method using

      ENV.cxx11 if build.cxx11?