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.
Add cmCommand::Disallowed helper to check the setting of a policy that
disallows the command. Add a RunCMake.DisallowedCommands test
placeholder. Add a Help/policy/DISALLOWED_COMMAND.txt file for
inclusion by each policy document to avoid duplication of the common
text.
Introduce a policy to control the behavior.
The AliasTargets unit test already tests that using a
double-semicolon in the name is not an error. Change the ExportImport
test to use a namespace with a double-semicolon too.
These policies were introduced after 2.8.12 in anticipation of 2.8.13.
However, we've now decided the next release will be 3.0.0, so update the
version of introduction accordingly.