THRIFT-3274 calling "make clean" twice in a row yields make error

fix tutorial
diff --git a/tutorial/cpp/Makefile.am b/tutorial/cpp/Makefile.am
index d8a9dbc..184a69d 100755
--- a/tutorial/cpp/Makefile.am
+++ b/tutorial/cpp/Makefile.am
@@ -71,7 +71,7 @@
 AM_LDFLAGS = $(BOOST_LDFLAGS) $(LIBEVENT_LDFLAGS)
 
 clean-local:
-	$(RM) -r gen-cpp
+	$(RM) gen-cpp/*
 
 tutorialserver: all
 	./TutorialServer