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

fusl/src/string/strspn.c

+#include <string.h>
+
+#define BITOP(a,b,op) \
+ ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))
+
+size_t strspn(const char *s, const char *c)
+{
+        const char *a = s;
+        size_t byteset[32/sizeof(size_t)] = { 0 };
+
+        if (!c[0]) return 0;
+        if (!c[1]) {
+                for (; *s == *c; s++);
+                return s-a;
+        }
+
+        for (; *c && BITOP(byteset, *(unsigned char *)c, |=); c++);
+        for (; *s && BITOP(byteset, *(unsigned char *)s, &); s++);
+        return s-a;
+}