Created Issue: build.fsx fails if used with with framework 4.0 msbuild.exe [87]

Logging a repro here for a bug report to microsoft and/or the open source version of f#.

Check out after [this changeset](https://rdotnet.codeplex.com/SourceControl/changeset/f8e3bd4164f0)

RDotNet.Release.sln solution builds, or cleans when done from VS2013, but not using msbuild version coming with the .NET framework

* Microsoft Visual Studio Professional 2013
* Version 12.0.30110.00 Update 1
* Microsoft .NET Framework
* Version 4.5.50938
* Visual F# 2013 <snip>
* Microsoft Visual F# 2013

```
C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe F:\codeplex\rdotnet\RDotNet.FSharp\RDotNet.FSharp.fsproj /t:Clean /m /p:Configuration="Release"
```

leads to:
```
F:\codeplex\rdotnet\RDotNet.FSharp\RDotNet.FSharp.fsproj : error MSB4057: The target "Clean" does not exist in the project.
```

whereas, replicating what VS2013 does under the hood, the following works:
```
"C:\Program Files (x86)\MSBuild\12.0\Bin\MSBuild.exe" F:\codeplex\rdotnet\RDotNet.Release.sln /t:Rebuild /m /p:Configuration="Release"
```