Allow -lz on Darwin

Change-Id: Ied37d4dca499f55e28494d1ce8eac59122827d67
diff --git a/cc/x86_darwin_host.go b/cc/x86_darwin_host.go
index 987b1dd..65a2dcc 100644
--- a/cc/x86_darwin_host.go
+++ b/cc/x86_darwin_host.go
@@ -89,6 +89,7 @@
 		"c",
 		"m",
 		"pthread",
+		"z",
 	}, "-l")
 )