Introduce `CMD_STRICT` to act as a parent for warnings about allowed but
disrecommended usage. Introduce a warning that discourages use of
non-target directives (which modify the build environment for the entire
[sub]tree). Add notes to the official documentation recommending use of
target-specific alternatives. This has been accepted wisdom for quite
some time, but only some of the affected commands made any mention of
this in the documentation.
Add new diagnostic categories CMD_POLICY and CMD_EXPERIMENTAL, both as
children of CMD_AUTHOR. Use the former when issuing policy warnings. Use
the latter when issuing experimental feature warnings.
Update documentation for aforementioned variables, as their use is
deprecated under the new diagnostic system. Introduce a policy which
reintroduces their use to control specific deprecation diagnostics if
the policy is not NEW.
Note that this means there is NO synchronization between the variables
and the new diagnostic state.
Make diagnostics into Sphinx objects so that they can be more readily
cross-referenced. (This is also needed to enable --help-diagnostic.)
Update various places that can make use of the same. Significantly
expand the documentation on controlling diagnostics.
Matthew Woehlke
Brad King