[xmonad] darcs patch: Separate source and build directories (and 1 more)

[Daniel Trstenjak]

> That's a natural problem when you try to put labels/tags on stuff and
> sort it into buckets (folders). It's often not so clear.

It's just annoying if people try hard to invent new names for stuff
which might be already there.

What should be the difference between tmp, cache and runtime?

> Using either XDG_CACHE_HOME or XDG_RUNTIME_DIR strikes me as
> preferable over ~/.xmonad_build.

I don't care that much which directory is used. The directory should
just have a defined behaviour. 

I don't think you want to rebuild xmonad on each login, because the
build files are deleted on each logout/reboot.