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/rb/Makefile b/test/rb/Makefile
index 502a981..326f720 100644
--- a/test/rb/Makefile
+++ b/test/rb/Makefile
@@ -19,4 +19,4 @@
 	ruby TestSuite.rb
 
 clean:
-	rm -fr gen-rb
+	$(RM) -r gen-rb