configure: better test for mktemp

Some variants of mktemp require a template, so provide one when
checking for the command.  We already supply a template in the
subsequent uses of mktemp.

Thanks to Michael Kostylev.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -1793,7 +1793,7 @@ HOSTEXESUF=$(exesuf $host_os)
 : ${TMPDIR:=$TMP}
 : ${TMPDIR:=/tmp}
 
-if ! check_cmd mktemp -u; then
+if ! check_cmd mktemp -u XXXXXX; then
     # simple replacement for missing mktemp
     # NOT SAFE FOR GENERAL USE
     mktemp(){