But it might be a minor problem for STB-style header libraries.
It's not uncommon for C++ projects to include the implementation of an STB-style header into a C++ source file instead of 'isolating' them in a C source file. That's about the only reason why I still support the common C/C++ subset in my C libraries.
Just tell them to go away.
Trying to write the subset of C and C++ is a fool's errand.