Fix a bug that would duplicate a path element in XDG_DATA_DIRS when
KDE_PREFIX == LOCALBASE. This triggered memory corruption in the xdgmime
code used by GTK+ and gnome-vfs which led to strange crashes in GTK+
applications such as Firefox and Thunderbird. While here, add a missing
XDG_DATA_DIRS entry for LOCALBASE/share/gnome.
Approved by: kde (lofi)