#1104 rpath correction

The previous solution was not tested. It did only work because
${CMAKE_INSTALL_PREFIX}/lib happened to be known to ld.

This commit is the proper way to do it as per
http://www.cmake.org/Wiki/CMake_RPATH_handling

Note: "There is a builtin CMake option CMAKE_SKIP_RPATH, if it is enabled all other RPATH related options are ignored"