The big blocker here is that a platform that fully supports C99 might not define the "NAN" macro. I don't think we can require that NAN be defined in order for Python to build (which is what the PR currently does, if I'm understanding it correctly).

Python deliberately doesn't assume IEEE 754 floating-point. By requiring that the C "NAN" macro is present to be able to build Python, we'd be effectively requiring IEEE 754 by stealth. (No other common floating-point format has NaNs.)

I'd be fully on board with a decision to require IEEE 754 floating-point for Python in future, but that decision would at least need a python-dev discussion - we shouldn't sneak it in by the back door.