contrib/jenkins.sh: execute distcheck/maintainer-clean with $PARALLEL_MAKE

Change-Id: I14b0e0343dc70b72cc9e14641bccaa6da2059e46
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 939a514..62e6516 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -58,12 +58,12 @@
   || cat-testlogs.sh
 LD_LIBRARY_PATH="$inst/lib" \
   DISTCHECK_CONFIGURE_FLAGS="--enable-vty-tests --enable-external-tests --enable-werror $CONFIG" \
-  $MAKE distcheck \
+  $MAKE $PARALLEL_MAKE distcheck \
   || cat-testlogs.sh
 
 if [ "$WITH_MANUALS" = "1" ] && [ "$PUBLISH" = "1" ]; then
 	make -C "$base/doc/manuals" publish
 fi
 
-$MAKE maintainer-clean
+$MAKE $PARALLEL_MAKE maintainer-clean
 osmo-clean-workspace.sh