Wip/targetver

This is for https://www.pivotaltracker.com/story/show/64552568 - Can't install on XP.

It sets the platform toolset to v110_xp so the resulting binaries can run on XP.

Also it changes the way how the `_WIN32_WINNT` and `_WIN32_IE` macros are handled for `targetver.h`. This comes now from a central place which is added to the include path in the props file.