[dwm] Fibonacci patch ported to 4.6

Some users requested the fibonacci patch, which was only for 4.4.1.
Even though I didn't use it, I tried to port it to 4.6, and it seems to work
fine. Please review it :)

Also, arg said earlier [1] that it should still be possible to keep a header file
(fibonacci.h in this case) and include it in config.h, instead of including
fibonacci.c directly.
I tried to do it, but failed :
dwm.c:(.text+0x2ada): undefined reference to `spiral'
dwm.c:(.text+0x2ae1): undefined reference to `dwindle'

It would be nice if someone could show me how to do this, I'm curious :)

[1] http://news.gmane.org/gmane.comp.window-managers.dwm