Support visual studio 2015 in Qt 4.8.7

Hi,

As the title says...

The current 4.8.7 build does not generate visual studio 2015 project files. It misses the makespec: "win32-msvc2015". So i tried to copy that from Qt 5.4.x. That surprisingly works and project files are being generated. It does annoy me with some warnings..

WARNING: /MP option is not supported in Visual C++ < 2005, ignoring.
WARNING: Could not parse Compiler option: -FS, added as AdditionalOption
WARNING: Could not parse Compiler option: -Zc:strictStrings, added as AdditionalOption

Those very same options work just fine when generating a visual studio 2010 project file.

Considering that 4.8.7 is supposed to be the last 4.8.x release and visual studio 2015 is going to be released this year, it would be great if this could make it in.