Bug 908984 - follow-up to bug 906316: submake launches need to know the new directory too

In this patch, we export the new XULRUNNER_DIRECTORY variable along with the
existing various xulrunner program paths. These paths have been made absolute
using make's `abspath` operation.

The patch also removes the ./ prefixes to those paths as they're likely useless
and prevent from using an absolute path when defining XULRUNNER_DIRECTORY from
the command line.