Revert RPATH to cmake's recommended setup

Without this patch, a build on Linux will produce executables (pdal, pdal_test) that can't find the shared libraries just one folder over in `lib/`. This patch nukes all Apple-specific RPATH management and instead implements the RPATH suggestions in http://www.cmake.org/Wiki/CMake_RPATH_handling. RPATH stuff was last tweaked in fc52dec. I've manually tested on both a Vagrant Ubuntu box and my home MacOSX.

This patch could also potentially be backported to **master** before merging -- let me know if you'd like me to do that.