THRIFT-4717: fix up make clean with autoconf
diff --git a/test/rb/Makefile.am b/test/rb/Makefile.am
index cfdc149..3910934 100644
--- a/test/rb/Makefile.am
+++ b/test/rb/Makefile.am
@@ -30,3 +30,8 @@
 	$(BUNDLER) exec $(RUBY) -I. test_suite.rb
 endif
 
+clean-local:
+	$(RM) -r gen-rb/
+
+dist-hook:
+	$(RM) -r $(distdir)/gen-rb/