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 "--c++11" option, and detect and enable C++11 support in
    install method using

      ENV.cxx11 if build.cxx11?

Closes #22453.
Clang standard library selection.

* Add new ENV function for selecting stdlib for Clang.
  - The selection is no-op for non-system-clang compilers.
  - Both superenv and stdenv are handled.
* Add new HOMEBREW_CCCFG flag and ccwrapper handling.
boost: support C++11 mode.

* Add info about option renaming: '--with-c++11' -> '--c++11'.
  - Require C++ dependencies with C++11 support when requesting '--c++11'.
* Building MPI python support in C++11 mode fails. Disable.
open-mpi: support C++11 mode.

* Disable silent rules for verbose build log.
* Include upstream patch to fix C++11 reserved string literal.

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

Closes #17750.
icu4c: support C++11 mode.

* Enable verbose build log.
yaml-cpp: support C++11 mode.

* Requires dependencies built with C++11 when requesting C++11 mode.

Closes #16162.
protobuf: support C++11 mode.

Closes #19141.
open-scene-graph: support C++11 mode.

Closes #19999.
geos: support C++11 mode.

Closes #22697.
Make stdenv handling less intrusive.

* Don't encode "-Qunused-arguments", as this is only supported in Clang
  and will break building when C/C++ compiler is switched during the
  process, e.g. building gcc. The warning is harmless without
  "-Werror".