Fix no -rt library on darwin.
diff --git a/configure.ac b/configure.ac
index e7ec8ca..f5246a7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -95,10 +95,12 @@ case $target in
CFLAGS="$CFLAGS -faltivec"
OPENCL_LIBS=""
PTHREAD_FLAGS=""
+ RT_LIBS=""
;;
*-*-darwin*)
OPENCL_LIBS="-framework OpenCL"
PTHREAD_FLAGS=""
+ RT_LIBS=""
;;
esac