option('zshcompletiondir', type : 'string',
description : 'directory for zsh completion scripts ["no" disables]')
-option('tests', type : 'combo', choices : ['true', 'unsafe'],
+option('tests', type : 'combo', choices : ['true', 'unsafe', 'false'],
description : 'enable extra tests with =unsafe')
option('slow-tests', type : 'boolean', value : 'false',
description : 'run the slow tests by default')