ext/POSIX/t/math.t: don't assume the sign of NaN

The test in Configure warns:

/* Note that whether the sign bit is on or off
 * for NaN depends on the CPU/FPU, and possibly
 * can be affected by the build toolchain.

but this test assumed that the default NaN was always positive,
but this isn't the case with the Sun/Oracle workshop cc, whether
on Oracle Linux or on Solaris.

copysign() is however well defined for NaN, so we can modify the
sign on NaN and test that with signbit().

Fixes Perl#21533

ext/POSIX/t/math.t

@@ -293,7 +293,8 @@ SKIP: {
     like(NAN, qr/^NaN/, "NAN is Perl's NaN");
     cmp_ok(NAN, '!=', NAN, "NAN != NAN");
     ok(!(NAN == NAN), "NAN == NAN");
-    ok(!signbit(NAN), "signbit(NAN)");
+    ok(!signbit(copysign(NAN, 1.0)), "signbit(copysign(NAN, 1.0)))");
+    ok(signbit(copysign(NAN, -1.0)), "signbit(copysign(NAN, -1.0)))");
 }
 
 done_testing();