(PCP-67) Set CMAKE_INSTALL_RPATH for OSX.
(PCP-67) Set CMAKE_INSTALL_RPATH for OSX.

This commit borrows from the CMakeLists.txt for Facter.

For OSX, we need to set the CMAKE_INSTALL_RPATH based
on the install prefix. Without this, pxp-agent fails to
find libcpp-pcp-client.so when run from the puppet-
agent install location on OSX.

MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client