Whether to use rpaths on Mac OS X.

When this property is set to true, the directory portion of the “install_name” field of shared libraries will be @rpath unless overridden by INSTALL_NAME_DIR. Runtime paths will also be embedded in binaries using this target and can be controlled by the INSTALL_RPATH target property. This property is initialized by the value of the variable CMAKE_MACOSX_RPATH if it is set when a target is created.

Policy CMP0042 was introduced to change the default value of MACOSX_RPATH to ON. This is because use of @rpath is a more flexible and powerful alternative to @executable_path and @loader_path.