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

Change-Id: I5fe776cf9ccb5d462c3c1fbbb1e31abe6f42bde6
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index eaf4f2b..8dc0162 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -36,7 +36,7 @@
 
 cd "$base"
 autoreconf --install --force
-./configure --enable-sanitize --enable-external-tests CFLAGS="-Werror" CPPFLAGS="-Werror"
+./configure --enable-sanitize --enable-external-tests --enable-werror
 $MAKE $PARALLEL_MAKE
 $MAKE check || cat-testlogs.sh
 $MAKE distcheck || cat-testlogs.sh