Add --with-c++11 switch to POCO formula

POCO by default compiles without C++11 support and gets linked with ``libstdc++``.
``--with-c++11`` switch enables building it with ``-std=c++11`` and ``-stdlib=libc++``.

There's also pull request for POCO that'll make it possible without patching in the future: https://github.com/pocoproject/poco/pull/83