Config settings without an argument

[henryiii]

Currently, build supports argumentless config-settings:

-Csomething

Is equivalent to

-Csomething=""

(since config settings is a dict in the spec). This is used, for example, to allow --config-setting=--with-c-extensions to be supported. However, pip does not support it.

I don't have a strong option on if this should be supported or not (I think I kind of like it?), but I think we should be consistent. What do people think here? If we like supporting this, I think we should ask pip to support it too. If we don't like supporting it (if it was added by mistake, for example), we could produce a warning when it's detected describing how to write it in a pip compatible way.

See pypa/cibuildwheel#1513 (comment)