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

fusl/src/stdio/ftell.c

+#include "stdio_impl.h"
+#include <limits.h>
+#include <errno.h>
+
+off_t __ftello_unlocked(FILE *f)
+{
+        off_t pos = f->seek(f, 0,
+                (f->flags & F_APP) && f->wpos > f->wbase
+                ? SEEK_END : SEEK_CUR);
+        if (pos < 0) return pos;
+
+        /* Adjust for data in buffer. */
+        return pos - (f->rend - f->rpos) + (f->wpos - f->wbase);
+}
+
+off_t __ftello(FILE *f)
+{
+        off_t pos;
+        FLOCK(f);
+        pos = __ftello_unlocked(f);
+        FUNLOCK(f);
+        return pos;
+}
+
+long ftell(FILE *f)
+{
+        off_t pos = __ftello(f);
+        if (pos > LONG_MAX) {
+                errno = EOVERFLOW;
+                return -1;
+        }
+        return pos;
+}
+
+weak_alias(__ftello, ftello);
+
+LFS64(ftello);