Making it compile on OS X

I made some changes to make the code compile on OS X, though I'm not entirely sure about them. 

The change in configure.ac allows for configure to fall back on libncurses instead of libncursesw. This fallback works in OS X, but in ubuntu it is indeed libncursesw that is required. I'm not familiar with these versions of the library, yet what I read is that the macports ncursesw has been [superseded](http://trac.macports.org/ticket/34872) by ncurses, or that the functionality which differentiates ncursesw from ncurses was integrated in ncurses.

The changes in src/ui/core.c are simply to satisfy the compiler. I have to admit I don't see the purpose of repeating the const keyword.

The changes regarding resource_presence_t in roster.c and presence.c seem to be straightforward and correct to me.