On OSX, don't build with install rpath
On OSX, don't build with install rpath

This is an identical change to #733. If cmake builds with install rpath
on OSX, `make test` is broken when `BUILD_SHARED_LIBS=ON` on OSX.

lucadelu/PDAL,lucadelu/PDAL,lucadelu/PDAL,lucadelu/PDAL