Add a c++14 configuration feature

qmake currently supports putting the compiler into C++11 mode by placing CONFIG += c++11 into the project file. This is implemented by the configuration feature file mkspecs/features/c++11.prf, and results in the -std=c++0x switch being passed to g++ or clang.

Clang 3.4 already supports the upcoming C++14 standard, and GCC 4.9.0 supports a useful subset. Both require the -std=c++1y switch to enable this mode. Visual Studio 2013 also supports a useful subset, and requires no switches to enable it.

It would be very useful to add a configuration feature to allow the use of CONFIG += c++14 in qmake project files, to cause the -std=c++1y switch to be passed to g++ or clang.