Make "make clean" rules a bit more standard.

Summary:
make-generic is some sort of internal undocumented thing.
make-local is what is supposed to be used for this stuff.
Also use $(RM) instead of "rm -f".

Reviewed By: marc

Test Plan: make clean


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665498 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/test/threads/Makefile b/test/threads/Makefile
index 58cdf18..59eb4f2 100644
--- a/test/threads/Makefile
+++ b/test/threads/Makefile
@@ -46,4 +46,4 @@
 	g++ -o ThreadsClient $(CFL) ThreadsClient.cpp ./gen-cpp/ThreadsTest.cpp ./gen-cpp/ThreadsTest_types.cpp
 
 clean:
-	rm -fr *.o ThreadsServer ThreadsClient gen-cpp gen-py
+	$(RM) -r *.o ThreadsServer ThreadsClient gen-cpp gen-py