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

fusl/src/math/ilogbl.c

+#include <limits.h>
+#include "libm.h"
+
+#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+int ilogbl(long double x)
+{
+        return ilogb(x);
+}
+#elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
+int ilogbl(long double x)
+{
+        #pragma STDC FENV_ACCESS ON
+        union ldshape u = {x};
+        uint64_t m = u.i.m;
+        int e = u.i.se & 0x7fff;
+
+        if (!e) {
+                if (m == 0) {
+                        FORCE_EVAL(0/0.0f);
+                        return FP_ILOGB0;
+                }
+                /* subnormal x */
+                for (e = -0x3fff+1; m>>63 == 0; e--, m<<=1);
+                return e;
+        }
+        if (e == 0x7fff) {
+                FORCE_EVAL(0/0.0f);
+                return m<<1 ? FP_ILOGBNAN : INT_MAX;
+        }
+        return e - 0x3fff;
+}
+#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
+int ilogbl(long double x)
+{
+        #pragma STDC FENV_ACCESS ON
+        union ldshape u = {x};
+        int e = u.i.se & 0x7fff;
+
+        if (!e) {
+                if (x == 0) {
+                        FORCE_EVAL(0/0.0f);
+                        return FP_ILOGB0;
+                }
+                /* subnormal x */
+                x *= 0x1p120;
+                return ilogbl(x) - 120;
+        }
+        if (e == 0x7fff) {
+                FORCE_EVAL(0/0.0f);
+                u.i.se = 0;
+                return u.f ? FP_ILOGBNAN : INT_MAX;
+        }
+        return e - 0x3fff;
+}
+#endif