Get build working in any command window
- Prefer VS 2013 (v12.0) version of MSBuild.exe and fallback to v4.0 when necessary
- Do not rely on %Path% since that limits us to VS developer cmd windows
- Match Microsoft.Build.Tasks*.dll version to MSBuild.exe, not type of cmd window
- Have tested building in VS 2013, VS 2013 dev cmd window, and bare cmd window

Requires either a FxCopCmd.exe.config binding redirect or David's "VS2013 FxCop
compatibility" change for full VS 2013 support.