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/Makefile.am b/test/Makefile.am
index db31738..f4dd05c 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -81,4 +81,4 @@
 AM_CPPFLAGS = $(BOOST_CPPFLAGS)
 
 clean-local:
-	rm -rf gen-cpp
+	$(RM) -r gen-cpp