back branches vs. VS 2008

The following patch allows me to build the 8.3 and 8.4 branches using 
Visual Studio 2008, once the build system is patched. But I don't really 
know why. HEAD and 9.0 build fine without it. But those branches 
branches fail with a complaint about IPPROTO_IPV6 being undefined.

The patch seems harmless enough. But I'd like to know why it's 
happening. Does anyone have a clue?

cheers

andrew

diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index 06aece3..c1775ea 100644
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -383,7 +383,7 @@ StreamServerPort(int family, char *hostName, 
unsigned short portNumber,        } #endif

-#ifdef IPV6_V6ONLY
+#if defined(IPV6_V6ONLY) && defined(IPPROTO_IPV6)        if (addr->ai_family == AF_INET6)        {            if
(setsockopt(fd,IPPROTO_IPV6, IPV6_V6ONLY,