#119 cabal new-build support

[George Thomas]

wxcore needs updates to its Setup.hs in order to work with the new style of cabal usage, because wxc.so ends up in a different location. I managed to get things working (Ubuntu 16, ghc 8.6.5, cabal 2.4.1, wxHaskell from GitHub) by adding the absolute path to wxc.so to extraLibDirs, removing the additions to ldOptions, and manually setting rpath for any executables making use of wxWidgets, but I'm sure a more principled solution could be found.