[go] fix func_backslash.mk and err_invalid_ifeq5.mk

emit "Nothing to be done for '<target>'" in these test.

don't remove "Nothing to be done for 'test' for err_invalid_ifreq3.mk
and mark it as TODO(c)
diff --git a/parser.go b/parser.go
index 8f85dd2..aef20e2 100644
--- a/parser.go
+++ b/parser.go
@@ -376,8 +376,7 @@
 	}
 	if len(extra) > 0 {
 		glog.V(1).Infof("extra %q", extra)
-		p.err = p.srcpos().errorf(`extraneous text after %q directive`, op)
-		return
+		warnNoPrefix(p.srcpos(), `extraneous text after %q directive`, op)
 	}
 
 	lhs, _, err := parseExpr([]byte(lhsBytes), nil, parseOp{matchParen: true})