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

fusl/src/math/frexp.c

+#include <math.h>
+#include <stdint.h>
+
+double frexp(double x, int *e)
+{
+        union { double d; uint64_t i; } y = { x };
+        int ee = y.i>>52 & 0x7ff;
+
+        if (!ee) {
+                if (x) {
+                        x = frexp(x*0x1p64, e);
+                        *e -= 64;
+                } else *e = 0;
+                return x;
+        } else if (ee == 0x7ff) {
+                return x;
+        }
+
+        *e = ee - 0x3fe;
+        y.i &= 0x800fffffffffffffull;
+        y.i |= 0x3fe0000000000000ull;
+        return y.d;
+}