Add fallback wcstod_l for musl

Just sets locale to "C" (because that's the only one we need), does
wcstod and resets the locale.

No idea why uselocale(loc) failed for me, but it did.

Fixes #5407.

src/fallback.cpp

@@ -388,3 +388,18 @@ int flock(int fd, int op) {
 }
 
 #endif  // HAVE_FLOCK
+
+#ifndef HAVE_WCSTOD_L
+// musl doesn't feature wcstod_l,
+// so we just wrap wcstod.
+double wcstod_l(const wchar_t *enptr, wchar_t **endptr, locale_t loc) {
+    char *saved_locale = strdup(setlocale(LC_NUMERIC, NULL));
+    // Yes, this is hardcoded to use the "C" locale.
+    // That's the only thing we need, and uselocale(loc) broke in my testing.
+    setlocale(LC_NUMERIC, "C");
+    double ret = wcstod(enptr, endptr);
+    setlocale(LC_NUMERIC, saved_locale);
+    free(saved_locale);
+    return ret;
+}
+#endif // defined(wcstod_l)