normalize path arguments to systemCwd

This seems necessary as a workaround so the build system works on both
Linux and OSX, since the two OSes interpret arguments to runProcess
differently (or something like that).

See https://github.com/haskell/cabal/commit/48082b90726093298df8559b365ed08805ca6d8f.
Merge branch 'master' of github.com:diagrams/diagrams-doc