jenkins.sh: ensure $MAKE is set

Change-Id: I2da8acdfe3abf79f68db4d00d04a7d162f0123ce
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 27cdaa9..7d3acc0 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -2,6 +2,11 @@
 
 set -ex
 
+if [ -z "$MAKE" ]; then
+  echo 'The $MAKE variable is not defined, cannot build'
+  exit 1
+fi
+
 if [ $sysmobts = "no" -a $sysmodsp = "yes" ]; then
    echo "This config does not make sense."
    exit 0