Review/mingw64-port

OCE now compiles smoothly with MinGW-w64.

Documentation-wise there is now a wiki entry on how to build with MinGW-w64 besides the new BUILD.MinGWw64 File.
These changes originate from this discussion on the mailing list: https://groups.google.com/forum/#!topic/oce-dev/KQVtNAmJgkY