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.
Brad King
Stephen Kelly
Nils Gladitz