POSIX: MSVC really does have copysign

This is enabled in config.gc, and gcc builds were using only msvcrt
for a long time, so presumably MSVC 2013 which uses msvcrt also
supports it.  Later MSVC uses ucrt which definitely supports
copysign.

win32/config.vc

@@ -133,8 +133,8 @@ d_class='undef'
 d_clearenv='undef'
 d_closedir='define'
 d_cmsghdr_s='undef'
-d_copysign='undef'
-d_copysignl='undef'
+d_copysign='define'
+d_copysignl='define'
 d_cplusplus='undef'
 d_crypt='define'
 d_crypt_r='undef'