Check if variable is set before adding to list

CMake/cotire.cmake

@@ -269,7 +269,9 @@ function (cotire_get_target_link_libraries_for_usage_requirements _target _targe
 			list (APPEND _targetLinkLibraries ${_library})
 			# process transitive libraries
 			get_target_property(_libraries ${_library} LINK_LIBRARIES)
-			list (APPEND _librariesToProcess ${_libraries})
+			if (_libraries)
+				list (APPEND _librariesToProcess ${_libraries})
+			endif()
 		endif()
 	endwhile()
 	set (${_targetLinkLibrariesVar} ${_targetLinkLibraries} PARENT_SCOPE)