Revert RPATH to cmake's recommended setup

Without this patch, executables in the build tree could not find shared
libraries in the build tree on Linux boxes. This patch removes
Apple-specific RPATH management and exactly implements the
recommendations from http://www.cmake.org/Wiki/CMake_RPATH_handling.
Merge pull request #410 from gadomski/linux-rpath

Revert RPATH to cmake's recommended setup