Enable build tree rpaths even with CMAKE_SKIP_RPATH
Enable build tree rpaths even with CMAKE_SKIP_RPATH

In order to generate the manual page documentation, the command line
utilities need to be run from the build tree, which means disabling
CMAKE_SKIP_RPATH.  CMAKE_INSTALL_RPATH is now unset if CMAKE_SKIP_RPATH
is true however, which gives an install tree free of rpaths as desired
for linux packaging.

jeremyselan/oiio,YangYangTL/oiio,lgritz/oiio,OpenImageIO/oiio,YangYangTL/oiio,scott-wilson/oiio,OpenImageIO/oiio,sambler/oiio,YangYangTL/oiio,scott-wilson/oiio,YangYangTL/oiio,micler/oiio,jeremyselan/oiio,sambler/oiio,sambler/oiio,bdeluca/oiio,mcanthony/oiio,micler/oiio,lgritz/oiio,mcanthony/oiio,bdeluca/oiio,mcanthony/oiio,jeremyselan/oiio,lgritz/oiio,cwilling/oiio,micler/oiio,lgritz/oiio,scott-wilson/oiio,bdeluca/oiio,bdeluca/oiio,scott-wilson/oiio,cwilling/oiio,sambler/oiio,cwilling/oiio,micler/oiio,OpenImageIO/oiio,mcanthony/oiio,OpenImageIO/oiio,cwilling/oiio