Allow for custom location of SandcastleConfigurator.vbs


As a best practice here, we put all of the custom MSBuild extensions in source control for each project we use. By doing this, we can get up and running without modifying multiple directories on a new box.
The only place $(MSBuildExtensionsPath) is used in Sandcastle.targets is when trying to execute the SandcastleConfigurator.vbs.
I propose that a new property be created, and use that instead to allow the Sandcastle extenstion to live anywhere you want, e.g.:
In the PropertyGroup:
<SandcastleTargetsPath Condition="'$(SandcastleTargetsPath)'==''">$(MSBuildExtensionsPath)\Sandcastle</SandcastleTargetsPath>
Later on:
<Target Name="CreateSandcastleConfig">
<Exec Command="WScript.exe "$(SandcastleTargetsPath)\SandcastleConfigurator.vbs" /in:"$(SandcastleOutputStructure)\Configuration\sandcastle.config" /out:sandcastle.config /ref:"$(ReferenceContentFilePath)" /shared:"$(SharedContentFilePath)" /output:"$(HelpOutputPath)" /path:"$(SandcastlePath)" "@(DocFiles, '" "')"" />