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?
Use HOMEBREW_CCCFG to handle C++11 mode.

* Use 'x' to mark C++11 mode.
* Use 'g' to mark Clang C++11 mode (for '-stdlib=libc++' handling).
* Detect :cxx and :cxxld and pass C++11 switches accordingly.
Rename option to "--c++11".
boost: support C++11 mode.
open-mpi: support C++11 mode.

* Also disable silent rules for verbose build log.

Closes #21452.
poco: support C++11 mode.

Closes #17750.