Fix building WXWIDGETS_2_8 with GHC 7.6.1

GHC 7.6.1 can't build WXWIDGETS_2_8 branch's wxdirect and wxcore. I made change to fix this problem.

I splited my changes into two part, their are general change ( shelarcy@5149eb3ba177efd305a243597adbd6090beef8c0 ) and WXWIDGETS_2_8 specific one. So, I think that we can pick general change to fix HEAD's same problem.

I already uploaded fixed version to HackageDB. Becase HackageDB's version didn't reflect c31ca7c11c974b05d4c0e7501d7f8d9b10d66e4f change, too. That causes building problem.

* http://sourceforge.net/tracker/?func=detail&atid=536845&aid=3572834&group_id=73133