jenkins: Enable tracing and error handling in bash

Like jenkins we want to see what is actually being executed
and fail if a subcommand has failed.
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 524fa8b..5c69da6 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 
+set -ex
+
 autoreconf --install --force
 ./configure --enable-static
 $MAKE $PARALLEL_MAKE