At least OpenBSD/sparc64 doesn't appear to define them, and possibly
some other OpenBSD GCC platforms don't (most OpenBSD platforms have
already switched to clang).
* See [Feature #17752]
* Defining explicitly to 0 seems the best solution, see https://github.com/ruby/ruby/pull/4428
* For example:
./include/ruby/internal/has/builtin.h:49:33: error: "RBIMPL_HAS_BUILTIN___builtin_assume" is not defined, evaluates to 0 [-Werror=undef]
49 | # define RBIMPL_HAS_BUILTIN(_) (RBIMPL_HAS_BUILTIN_ ## _)
| ^~~~~~~~~~~~~~~~~~~
./include/ruby/internal/assume.h:75:7: note: in expansion of macro ‘RBIMPL_HAS_BUILTIN’
75 | #elif RBIMPL_HAS_BUILTIN(__builtin_assume)
| ^~~~~~~~~~~~~~~~~~