[Bug target/126782] [Darwin] sys/_types/_rsize_t.h does not define rsize_t with -fmodules

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126782

--- Comment #3 from Drea Pinski <pinskia at gcc dot gnu.org> ---
Fixincludes are discouraged on darwin/mac because it is about the specific sdk
and you can supply different SDKs at build time.

What is USE_CLANG_STDDEF even support to do here?