Write rpaths when installing

Created by: jmcarcell

BEGINRELEASENOTES

• Write rpaths when installing. Fixes an issue introduced in https://github.com/iLCSoft/iLCUtil/pull/32 where rpaths are not being written.

ENDRELEASENOTES

Before https://github.com/iLCSoft/iLCUtil/pull/32 and after for non-Apple platforms are not equivalent because before rpaths are being written and after they are not. I believe the default value of CMAKE_SKIP_INSTALL_RPATH is FALSE. With this change I can see rpaths are being written again.