option('log_backend', type: 'combo',
  choices: ['stderr', 'spdlog', 'loguru', 'plog'],
  value: 'stderr',
  description: '''The logging backend to use. This option can also be set via
-DLIBERATE_LOG_BACKEND=`...`. Possible values are `stderr` (the default),
`spdlog`, `loguru` and `plog`. See README.md for logging detail.''',
)
option('build_extras', type: 'boolean',
  value: true,
  description: '''Build extras such as tests, examples, etc.'''
)