jenkins.sh: use --enable-werror configure flag, not CFLAGS

Change-Id: I487ab6f2e391aa57404725de2c8ab5447ca9f725
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 45e540e..6a76044 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -33,7 +33,7 @@
 set -x
 
 autoreconf --install --force
-./configure --enable-sanitize CFLAGS="-Werror" CPPFLAGS="-Werror"
+./configure --enable-sanitize --enable-werror
 $MAKE $PARALLEL_MAKE
 $MAKE distcheck \
   || cat-testlogs.sh