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

Change-Id: I4791c6a32d983869692df36172f93adee819debb
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 5f31710..7d68736 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -13,7 +13,7 @@
 set -x
 
 autoreconf --install --force
-./configure CFLAGS="-Werror" CPPFLAGS="-Werror"
+./configure --enable-werror
 $MAKE $PARALLEL_MAKE
 $MAKE distcheck \
   || cat-testlogs.sh