[josephg]

> In practice, build systems need to cope with systems that pretend to be other systems via incompletely-implemented compatibility layers.

That sounds fine though. If the system claims to provide feature X, you probably want the program in question to compile assuming feature X is available. If the compatibility layer doesn’t work as advertised, a compiler error is a great choice. Let the user choose to turn off that flag in their system configuration when building the project.

I’m not proposing user agent sniffing. I’m proposing something much more fine grained than that. Make something that looks more like the output of configure that build systems can use as input.