Copy cmGlobalVisualStudio11Generator to cmGlobalVisualStudio12Generator
and update version numbers accordingly.  Add the VS12 enumeration value.
Add module CMakeVS12FindMake to find MSBuild.  Look for MSBuild in its
now-dedicated Windows Registry entry.  Teach the platform module
Windows-MSVC to set MSVC12 and document the variable.  Teach module
InstallRequiredSystemLibraries to look for the VS 12 runtime libraries.

Teach tests CheckCompilerRelatedVariables, Preprocess, VSExternalInclude,
and RunCMake.GeneratorToolset to treat VS 12 as they do VS 10 and 11.

Inspired-by: Minmin Gong <minmin.gong@gmail.com>