Add -c++14 flag (or equivalent change) to configure script

At the moment it is possible to pass the option “-c++11” to the configure script to cause Qt to be built with C++11 support. I assume that (importantly) this also passes the appropriate compiler options /while building Qt/, allowing Qt code to take advantage of C++11 features when available.

A similar option should be added to support building Qt using the compiler in C++14 mode. Clearly[1] Qt is already starting to use C++14 features, and so it should be possible to configure the build of Qt so that it runs the compiler in a way which will take advantage of them while /building/ Qt!

Alternatively (perhaps better but also perhaps more work), the “-c++11” flag for the configure script should be deprecated and a “-c++version=” flag should be added, so I can say “-c++version=11”, “-c++version=14”, etc.

Aside: for current Apple Clang, the required flag is “-std=c++1y”.

(would have added labels c++14, configure, build, buildsystem, … but I'm not allowed to add labels!

[1] http://woboq.com/blog/cpp14-in-qt.html