gsu_make_tempfile(): Avoid reading an unbound variable.

If a gsu script

	(a) enables the -u option to treat unset variables as
	    an error for parameter expansion,
	(b) does not set TMPDIR,
	(c) calls gsu_make_tempfile(),

gsu_make_tempfile() fails with an "TMPDIR: unbound variable" error.

The code already contains a fallback for this case, so we can use
the :- construct to avoid the error and fall back to /tmp if TMPDIR
is unset or NULL.