VS: Corrected usaged of std::string append.
Merge topic 'vs2013_toolset_14369' into next

e10ceec VS: Corrected usaged of std::string append.
VS: Each Visual Studio Generator has a tools version.

The MSBuild version for each visual studio generator isn't 4.0. With Visual
Studo 2013 the ToolsVersion moved from being tied to the .Net Generator and now
has its own version number.
Merge topic 'vs2013_toolset_14369' into next

f0cf64c VS: Each Visual Studio Generator has a tools version.