Add a "fork" of musl as //fusl.

fusl/src/locale/setlocale.c

+#include <locale.h>
+#include <stdlib.h>
+#include <string.h>
+#include "locale_impl.h"
+#include "libc.h"
+#include "atomic.h"
+
+static char buf[LC_ALL*(LOCALE_NAME_MAX+1)];
+
+static char *setlocale_one_unlocked(int cat, const char *name)
+{
+        const struct __locale_map *lm;
+
+        if (name) libc.global_locale.cat[cat] = lm = __get_locale(cat, name);
+        else lm = libc.global_locale.cat[cat];
+
+        return lm ? (char *)lm->name : "C";
+}
+
+char *__strchrnul(const char *, int);
+
+char *setlocale(int cat, const char *name)
+{
+        static volatile int lock[2];
+
+        if ((unsigned)cat > LC_ALL) return 0;
+
+        LOCK(lock);
+
+        /* For LC_ALL, setlocale is required to return a string which
+         * encodes the current setting for all categories. The format of
+         * this string is unspecified, and only the following code, which
+         * performs both the serialization and deserialization, depends
+         * on the format, so it can easily be changed if needed. */
+        if (cat == LC_ALL) {
+                int i;
+                if (name) {
+                        char part[LOCALE_NAME_MAX+1] = "C.UTF-8";
+                        const char *p = name;
+                        for (i=0; i<LC_ALL; i++) {
+                                const char *z = __strchrnul(p, ';');
+                                if (z-p <= LOCALE_NAME_MAX) {
+                                        memcpy(part, p, z-p);
+                                        part[z-p] = 0;
+                                        if (*z) p = z+1;
+                                }
+                                setlocale_one_unlocked(i, part);
+                        }
+                }
+                char *s = buf;
+                for (i=0; i<LC_ALL; i++) {
+                        const struct __locale_map *lm =
+                                libc.global_locale.cat[i];
+                        const char *part = lm ? lm->name : "C";
+                        size_t l = strlen(part);
+                        memcpy(s, part, l);
+                        s[l] = ';';
+                        s += l+1;
+                }
+                *--s = 0;
+                UNLOCK(lock);
+                return buf;
+        }
+
+        char *ret = setlocale_one_unlocked(cat, name);
+
+        UNLOCK(lock);
+
+        return ret;
+}