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

[Daniel Trstenjak]

> Which directory is defined to be cleared on each login?

XDG_RUNTIME_DIR: "The lifetime of the directory MUST be bound to the user being logged in." [1]

That's not explicitly demanded for XDG_CACHE_HOME, but some systems
might still do it, because of the "non-essential" data part, or put it
for performance reasons on a ram disk, like they do with /tmp.