This is Microsoft.Build(XBuild) implementation. It will consist of:

Microsoft.Build.Engine
Microsoft.Build.Framework
Microsoft.Build.Tasks
Microsoft.Build.Utilities

There are .targets files that can be used to build XBuild. To do it you must have finished build in xbuild/bin/Debug.
Microsoft.Build.csproj is not a regular VS2005 project.

It is licensed on MIT X11 license.