Avoid passing -std=c++0x when it is not supported

Extend the check in KDECompilerSettings.cmake to only pass -std=c++0x to the
compiler if it's recent enough, otherwise we risk passing that to base GCC and
breaking the build of any extra-cmake-modules-dependent port using
KDECompilerSettings.cmake on FreeBSD 9 (such as multimedia/qt5-phonon4).

PR: 212427
Submitted by: rakuco
Approved by: yuri@rawbw.com (maintainer)