MSBuildHelper BuildWebsite fails with invalid OutDir path

[reidev275]

This happens because of the prefix code in BuildWebsite https://github.com/fsharp/FAKE/blob/master/src/app/FakeLib/MSBuildHelper.fs#L435.

My outputPaths are local file paths like c:\deploys\artifacts so the existing ../ prefix turns my output paths into ../../c:\deploys\artifacts

If I switch the prefix code to check if the path is rooted the build works.

let prefix = if Path.IsPathRooted outputPath
             then ""
             else (String.replicate diff "../")

I may be missing something with the initial intent of the prefix code that would break with this suggested change though.

@forki since you had the initial commit on this line can you help determine whether my suggestion helps or hurts?

[matthid]

Seems to be fixed.