#88 Files are always generated and compiled, when running cabal install

[Henk-Jan van Tuyl]

When I compile my wxHaskell application in a cabal sandbox with local wxHaskell source, the following files are generated and compiled each time (leading to other sources to be compiled as well):
- WxcClassInfo.hs
- WxcClassTypes.hs
- WxcClasses.hs
- WxcClassesAL.hs
- WxcClassesMZ.hs

When a check on modification times is added to wxcore, the compile time of the application will be reduced considerably.