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

[Daniel Trstenjak]

> An alternative location could be ~/.cache/xmonad.
> yi - possibly via dyre - stores stuff in ~/.cache/yi.

Yes, I thought about it, but only putting the build files in a
freedesktop conforming directory could be perhaps confusing, and I
didn't wanted to change the configuration/source directory for backward
compatibility reasons.

Perhaps having the search order ["~/.config/xmonad", "~/.xmonad"] for
configuration/source files could solve that issue.

Looking at the 'XDG Base Directory Specification'[1], I don't know if
the cache directory is the right one, because of: "... user-specific
>>> non-essential <<< (cached) data should be written."

It's also not that clear what the difference is between $XDG_CACHE_HOME and
$XDG_RUNTIME_DIR.

There's also $XDG_DATA_HOME, which default value is "~/.local/share",
which seems quite strange for user specific data.

The XDG seems to be the kind of specification which might create more
issues than it solves.

[1] http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html