[dev] wmii-hg build fixes

I recently tried to build wmii-hg on my funtoo laptop, but was met with a
multitude of `undefined reference to blah' errors.

I hacked together a fix that allows wmii-hg to compile on my machine. I had to
add some things to INCLUDE and LIBS. I also ifdef'd the xinerama code that I
could find and added XINERAMALIBS and XINERAMAFLAGS to config.mk. You can enable
Xinerama by uncommenting those; just like in dwm.

I'm sure there's a better way to do that, but it's all I could think of atm.
I've attached the diff with the changes I've made.

-- 
I am a man who does not exist for others.