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

fusl/src/math/roundl.c

+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+long double roundl(long double x)
+{
+        return round(x);
+}
+#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
+
+static const long double toint = 1/LDBL_EPSILON;
+
+long double roundl(long double x)
+{
+        union ldshape u = {x};
+        int e = u.i.se & 0x7fff;
+        long double y;
+
+        if (e >= 0x3fff+LDBL_MANT_DIG-1)
+                return x;
+        if (u.i.se >> 15)
+                x = -x;
+        if (e < 0x3fff-1) {
+                FORCE_EVAL(x + toint);
+                return 0*u.f;
+        }
+        y = x + toint - toint - x;
+        if (y > 0.5)
+                y = y + x - 1;
+        else if (y <= -0.5)
+                y = y + x + 1;
+        else
+                y = y + x;
+        if (u.i.se >> 15)
+                y = -y;
+        return y;
+}
+#endif