Update documentation to account for commit ae5f40696e (CMP0037: Allow
test and package targets when features are not enabled, 2017-10-26).
Issue: #16062
The most likely documentation page a project author will read in
response to a policy warning is the page for the policy itself.
Add to every policy documentation page a note explicitly stating
that the OLD behavior is deprecated. Also mention this in the
cmake_policy() command documentation that explains how to set a
policy to OLD.
Suggested-by: Fraser Hutchison <fraser.hutchison@gmail.com>
Starting with 3.0 we will use only two components for the feature level,
and policies are only ever introduced with a bump to the feature level
version.
Teach add_custom_target to check the policy too. Extend the policy to
disallow reserved target names that we use for builtin targets like
"all".
Extend the RunCMake.CMP0037 test to cover these cases.
