Override the ToolsVersion Settings of Projects on Command Line Builds

Although Visual Studio 2008 projects typically build with the
ToolsVersion specified in the project file, you can use the
/ToolsVersion (or, /tv) switch on the command line to override that
value and build all of the projects and their project-to-project
dependencies with a different Toolset.

C:\windows\microsoft.net\framework\v3.5>msbuild.exe someproj.proj
/tv:3.5 /p:Configuration=Debug