Support XDG_RUNTIME_DIR being unset

[Emilio Pozuelo Monfort]

Hi,

The XDG base directory specification[1] says that "If $XDG_RUNTIME_DIR
is not set applications should fall back to a replacement directory
with similar capabilities and print a warning message."

Currently we abort if XDG_RUNTIME_DIR is not set. The following patches
fall back to ~/.cache, as GLib does.

Note that I haven't made wl_get_runtime_dir() public, and so I had to
copy it into weston.

Cheers,
Emilio

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