Adjust the Thrift compiler makefile to clean up after the obj/ folder that it builds...

Adjust the Thrift compiler makefile to clean up after the obj/ folder that it builds into

Summary: I am super nitpicky, and after make clean I don't like seeing:
$ svn st
?   obj/

Problem: make clean didn't remove the generated obj/ dir

Solution: Kill it!

Reviewed By: aditya

Test Plan: Do make clean in compiler directory followed by svn st. You should see nothing!

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@664717 13f79535-47bb-0310-9956-ffa450edef68