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.