Allow libdl on Darwin

Fixes the build, since this is available.

Change-Id: Ia4f50af234ee0f4ae5c7beb54724f590f2835db5
diff --git a/cc/x86_darwin_host.go b/cc/x86_darwin_host.go
index a0643ef..2894f7e 100644
--- a/cc/x86_darwin_host.go
+++ b/cc/x86_darwin_host.go
@@ -79,6 +79,7 @@
 
 	darwinAvailableLibraries = addPrefix([]string{
 		"c",
+		"dl",
 		"m",
 		"pthread",
 		"z",