Should this use CHECK_SYMBOL_EXISTS(regcomp_l "xlocale.h" HAVE_REGCOMP_L) instead?

Yes, that's what it looks like from its documentation. I wonder if CHECK_FUNCTION_EXISTS might work here as well, since it's system-provided.

If (locale_t) 0 defined somewhere to be special? I've found LC_GLOBAL_LOCALE in the headers which is defined as (locale_t) -1 but not for 0.

Unfortunately, I can't find anything either.

Initially, I created a new locale through newlocale(LC_ALL_MASK, "C", (locale_t) 0);, but I tested using (locale_t) 0 directly and that seems to work as well.

We could change it back to using newlocale and storing that locale somewhere if you think that'd be safer?

From Apple's macOS source code release:

int
tre_regncomp_l(regex_t *preg, const char *regex, size_t n, int cflags, locale_t loc)
{
  ...

  NORMALIZE_LOCALE(loc);

  ...
}

#define NORMALIZE_LOCALE(x) if ((x) == NULL) { \
    (x) = _c_locale; \
} else if ((x) == LC_GLOBAL_LOCALE) { \
    (x) = &__global_locale; \
}

So we should be fine passing (locale_t) 0 (or even just NULL) here. Do you want me to change anything here?

newlocale does mention LC_GLOBAL_LOCALE as not being a valid input as the base, but (locale_t) 0 is, so I suppose it's fine to leave it with 0.