[HEADSUP][CFT] New compiler USES flag, please test, review comment

[Robert Burmeister]

> USES= compiler:c++11
>
> Means you need a c++1 capable compiler but not necessary a c++ 11 aware standard
> library, in that case if the base compiler is not able to support c++11 clang33
> will be used from ports otherwise gcc 4.6+ if you specify it in make.conf.
Problem, if clang from ports is >3.3.

http://freebsd.1045724.n5.nabble.com/FreeBSD-Port-editors-libreoffice-td5856164.html