OS X: Add support for @rpath in export files.

Also expand the IMPORTED_SONAME property for targets
to match the install_name.

Source/cmExportBuildFileGenerator.h

@@ -61,6 +61,8 @@ protected:
                                  cmTarget* target,
                                  ImportPropertyMap& properties);
 
+  std::string InstallNameDir(cmTarget* target, const std::string& config);
+
   std::vector<cmTarget*> const* Exports;
   cmExportCommand* ExportCommand;
 };