[Suggestion] Alert single-config generator users when they use --config

This post highlights a confusion for new cmake users.

They are using --config on a single configuration generator build. Which of course does nothing. Since --config is for multi-configuration generators.

How can we avoid this confusion for new users?

I think it would be reasonable to warn from cmake --build . --config $config on a single-configuration generator if $config does not match its CMAKE_BUILD_TYPE.

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