policy: relax the default for in-place build We're going to make the 'c' policy more strict, where no missing attribute will be allowed. Since we want to run 'hg bisect' without rebuilding the C extension modules, we'll need a looser policy for development environment. The default for system installation isn't changed. Note that the current 'c' policy is practically 'allow'-ish as we have lots of adhoc fallbacks to pure functions.