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

Change-Id: I64e542ad4da34a7ac3bc1b599a122ecff47e892d
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 033eed7..19df974 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -38,7 +38,7 @@
 
 cd "$base"
 autoreconf --install --force
-./configure --enable-sanitize CFLAGS="-Werror" CPPFLAGS="-Werror" $GTP
+./configure --enable-sanitize --enable-werror $GTP
 $MAKE $PARALLEL_MAKE
 $MAKE distcheck