POSIX: MSVC really does have signbit()

But it appears that the fallback for Perl_signbit() in numeric.c
is broken when we've found a definition for Perl_fp_class_nzero().

When Perl_fp_class_nzero is defined, the fallback Perl_signbit()
simply returns that value, but Perl_fp_class_nzero() only returns
true when the number is negative zero, not for any negative number.

But C99 requires signbit(), so I don't think it's worth spending
any more time looking into this.  We might want to eliminate
Perl_signbit() from numeric.c, or try to fix it.

win32/config.vc

@@ -529,7 +529,7 @@ d_siginfo_si_pid='undef'
 d_siginfo_si_status='undef'
 d_siginfo_si_uid='undef'
 d_siginfo_si_value='undef'
-d_signbit='undef'
+d_signbit='define'
 d_sigprocmask='undef'
 d_sigsetjmp='undef'
 d_sin6_scope_id='define'