handle installation without RPATH on UNIX properly

handle installation without RPATH on UNIX properly

the custom ld.so.conf file ensures that the Katie libraries get
registered in the cache so that when installed in location other than
/usr/lib or other standard libraries path executables can operate.

Even the default libraries path (/usr/local/lib/katie) is
non-standard and requires the config, unless CMAKE_SKIP_INSTALL_RPATH
if set to ON but because RPATH is considered security issue it is
unlikely that binary distributors will turn it on when building Katie

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>