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/lib/py/Makefile.am b/lib/py/Makefile.am
index 1142b3c..a91d28c 100644
--- a/lib/py/Makefile.am
+++ b/lib/py/Makefile.am
@@ -11,7 +11,7 @@
 install-exec-hook:
 	$(PYTHON) setup.py install --root=$(DESTDIR) --prefix=$(PY_PREFIX) $(PYTHON_SETUPUTIL_ARGS)
 
-clean-generic:
-	rm -rf build
+clean-local:
+	$(RM) -r build
 
 check-local: all