Amending PEP 517 with a sentence on handling duplicate config-settings keys

[steve.dower]

@pf_moore

@mgorny

I can’t imagine how you’d be able to use config_settings more generically, given that there are no “standard” values and every backend can define their own (and throw an error on unsupported keys).

The hope when the PEP was written was that backends would agree between themselves on how they would use config_settings. In hindsight, I guess that was naïve

It may still happen, but I think the bigger issue is that some config_settings are unlikely to apply to all packages involved in an installation. So you end up needing a syntax for “when installing package X, use this config, otherwise ignore it”.

We started approaching some agreement on cross-compiling settings, which ought to apply to all packages with native content, but then because pure-Python only backends would throw on them it wouldn’t actually help. Best to use the backend directly for that package to build a wheel, and then only install from wheels.