[go] fix override_export.mk

separete override_export.mk and create err_export_override.mk and
err_override_export.mk
diff --git a/parser.go b/parser.go
index 84f633a..059688f 100644
--- a/parser.go
+++ b/parser.go
@@ -564,6 +564,7 @@
 	line = append(line, []byte("override ")...)
 	line = append(line, data...)
 	p.handleRuleOrAssign(line)
+	// TODO(ukai): evaluate now to detect invalid "override" directive here?
 }
 
 func handleExport(p *parser, data []byte, export bool) (hasEqual bool) {
@@ -576,10 +577,10 @@
 		}
 		data = data[:i]
 	}
-
 	east := &exportAST{
-		expr:   data,
-		export: export,
+		expr:     data,
+		hasEqual: hasEqual,
+		export:   export,
 	}
 	east.srcpos = p.srcpos()
 	glog.V(1).Infof("export %v", east)