contrib: jenkins: Enable parallel make in make distcheck

Related: OS#4421
Change-Id: I4db992d599bd5cdc16a58729695654d1b759b682
diff --git a/contrib/jenkins_amd64.sh b/contrib/jenkins_amd64.sh
index c79e26a..3fee57c 100755
--- a/contrib/jenkins_amd64.sh
+++ b/contrib/jenkins_amd64.sh
@@ -29,7 +29,7 @@
 # do distcheck only once, which is fine from built source tree, since distcheck
 # is well separated from the source tree state.
 DISTCHECK_CONFIGURE_FLAGS=--enable-external-tests \
-    $MAKE distcheck \
+    $MAKE $PARALLEL_MAKE distcheck \
     || cat-testlogs.sh
 $MAKE maintainer-clean