qOverload not defined for MSVC compiler

The qOverload (along qConstOverload and qNonConstOverload) is not defined for Visual Studio compiler, even though it supports variable templates (since 2015 Update 3 I believe).

It seems that qglobal.h defines qOverload only if __cpp_variable_templates feature detection macro is defined. (line 1100 as of Qt 5.9.0) The feature detection macros are not part of C++ standard and MSVC currently does not define them. I believe that there should be added check for MSVC version as well.