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

fusl/src/math/modfl.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double modfl(long double x, long double *iptr)
+{
+        double d;
+        long double r;
+
+        r = modf(x, &d);
+        *iptr = d;
+        return r;
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+
+static const long double toint = 1/LDBL_EPSILON;
+
+long double modfl(long double x, long double *iptr)
+{
+        union ldshape u = {x};
+        int e = (u.i.se & 0x7fff) - 0x3fff;
+        int s = u.i.se >> 15;
+        long double absx;
+        long double y;
+
+        /* no fractional part */
+        if (e >= LDBL_MANT_DIG-1) {
+                *iptr = x;
+                if (isnan(x))
+                        return x;
+                return s ? -0.0 : 0.0;
+        }
+
+        /* no integral part*/
+        if (e < 0) {
+                *iptr = s ? -0.0 : 0.0;
+                return x;
+        }
+
+        /* raises spurious inexact */
+        absx = s ? -x : x;
+        y = absx + toint - toint - absx;
+        if (y == 0) {
+                *iptr = x;
+                return s ? -0.0 : 0.0;
+        }
+        if (y > 0)
+                y -= 1;
+        if (s)
+                y = -y;
+        *iptr = x + y;
+        return -y;
+}
+#endif