This prevents the following test errors on Windows:

   perf/haddock haddock.Cabal [[Errno 2] No such file or directory: ...
   perf/haddock haddock.base [[Errno 2] No such file or directory: ...
   perf/haddock haddock.compiler [[Errno 2] No such file or directory: ...

The tests will now be reported as having missing libraries.