Looks like we can't declare getpeereid on Windows anyway.

tglsfdc · tglsfdc · commit 680ea6a6df34 · 2011-06-02T17:27:30.000-04:00
... for lack of the uid_t and gid_t typedefs.  Per buildfarm.
diff --git a/src/include/port.h b/src/include/port.h
@@ -395,7 +395,7 @@ extern void srand48(long seed);
 extern int	getopt(int nargc, char *const * nargv, const char *ostr);
 #endif
 
-#ifndef HAVE_GETPEEREID
+#if !defined(HAVE_GETPEEREID) && !defined(WIN32)
 extern int	getpeereid(int sock, uid_t *uid, gid_t *gid);
 #endif
 
