mac rpath support pre-cmake 2.8.12

From bruen...@google.com on February 23, 2014 00:33:07

If we move to cmake 2.8.12 we can use its built-in rpath support ( http://www.kitware.com/blog/home/post/510 )

Until then we add @rpath ourselves. The downside on Mac is that everyone who uses this lib has to have the rpath set and can't just copy the lib to the local dir (should we document that? DynamoRIO_RPATH is on for standalone, which covers most uses of drconfiglib and drinjectlib, the only libs I really need to do this for).

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1375