[Qt5] Remove 'c++11' USE flag.

Let qtbase's configure automatically enable C++11 if
supported by the current platform/compiler combination.
[Qt4] Drop support for USE='c++0x' on 4.8.5 and later.

Upstream confirmed that there's no point in building Qt4 with
C++11 enabled. Real benefits in the libs came only with Qt5.
[qt4-build.eclass] Don't die on downgrade, just warn the user.