Both Hadoop 1.X and Hadoop 2.X have deprecated the usage of HADOOP_HOME. It is suggested that the components not rely on it directly and instead autodetect hadoop location. Most of the Hadoop ecosystem components (pig, hive, hbase, mahout, etc) have introduced such modifications into their launcher scripts. A good example for Giraph to follow would be HBase (start at line #234):
It would be highly desirable for Giraph to follow. If there's an agreement I can provide a trivial patch.