Add Q_DECL_RELAXED_CONSTEXPR macro

Expands to constexpr if C++14 relaxed constepxr are supported by the compiler

[ChangeLog][QtCore] Added Q_DECL_RELAXED_CONSTEXPR for the corresponding C++14 feature

Change-Id: Id6b56b3a17da2ff838c80795e528e1247dc13f63
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>