Partially revert commit 536d47bd.

Per buildfarm, the "#ifdef F_SETFD" removed in that commit actually
is needed on Windows, because fcntl() isn't available at all on that
platform, unless using Cygwin.  We could perhaps spell it more like
"#ifdef HAVE_FCNTL", or "#ifndef WIN32", but it's not clear that
those choices are better.

It does seem that we don't need the bogus manual definition of
FD_CLOEXEC, though, so keep that change.

Discussion: https://postgr.es/m/26254.1492805635@sss.pgh.pa.us

src/interfaces/libpq/fe-connect.c

@@ -1983,6 +1983,7 @@ keep_going:						/* We will come back to here until there is
 						continue;
 					}
 
+#ifdef F_SETFD
 					if (fcntl(conn->sock, F_SETFD, FD_CLOEXEC) == -1)
 					{
 						appendPQExpBuffer(&conn->errorMessage,
@@ -1992,6 +1993,7 @@ keep_going:						/* We will come back to here until there is
 						conn->addr_cur = addr_cur->ai_next;
 						continue;
 					}
+#endif   /* F_SETFD */
 
 					if (!IS_AF_UNIX(addr_cur->ai_family))
 					{