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

fusl/src/math/nexttoward.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+double nexttoward(double x, long double y)
+{
+        return nextafter(x, y);
+}
+#else
+double nexttoward(double x, long double y)
+{
+        union {double f; uint64_t i;} ux = {x};
+        int e;
+
+        if (isnan(x) || isnan(y))
+                return x + y;
+        if (x == y)
+                return y;
+        if (x == 0) {
+                ux.i = 1;
+                if (signbit(y))
+                        ux.i |= 1ULL<<63;
+        } else if (x < y) {
+                if (signbit(x))
+                        ux.i--;
+                else
+                        ux.i++;
+        } else {
+                if (signbit(x))
+                        ux.i++;
+                else
+                        ux.i--;
+        }
+        e = ux.i>>52 & 0x7ff;
+        /* raise overflow if ux.f is infinite and x is finite */
+        if (e == 0x7ff)
+                FORCE_EVAL(x+x);
+        /* raise underflow if ux.f is subnormal or zero */
+        if (e == 0)
+                FORCE_EVAL(x*x + ux.f*ux.f);
+        return ux.f;
+}
+#endif