[xbuild] fix 4.0-only test condition.
[MS.Build] add MS.Build.dll-specific well-known property, to differentiate from MS.Build.Engine.
[msbuild/xbuild] use ToolLocationHelper.GetPathToStandardLibraries() to get corlib path, for new build engine.

This is enabled only with Microsoft.Build.dll because property function
support is not fully implemented in MS.Build.Engine.dll and simply fails.

Since the targets files are shared between xbuild and msbuild, I added
some well-known property for msbuild and used it in the targets file.