Deprecate the C++14 compiler feature macros

There's now a C++ standards Standing Document that defines preprocessor
macros in the same way that we used to. So we no longer need to define
them ourselves. The current macros are kept for compatibility purposes,
for the compilers where they used to be defined.

The list will not be extended with new macros or for new compiler
versions.

[ChangeLog][Deprecation Notice] The Q_COMPILER_xxx macros for C++14
compiler features introduced in Qt 5.3 are deprecated and will not be
updated for new compilers either. User code should be changed to use the
macros from Standing Document 6 instead:
http://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations.
This does not affect C++11 feature macros.

Change-Id: I246afb84263f3d7ff72ccc0bc44bf86a6fc7cd96
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>