...is actually very uncommon in the C world (at least much less common than in the C++ or Rust world).

Specific compilers and stdlibs have specific behaviours which may or may not fully agree with the details written down in the standard. You'll have to pick a subset of compilers you want to support and write your code against that subset of compilers. It's not perfect, but also not much of a problem. It's simply the reality when there are multiple compiler implementations.