Remove configure probe for net/if.h.

<net/if.h> is in SUSv3 and all targeted Unixes have it.  It's used in a
region that is already ifdef'd out for Windows.  We're not using it for
any standard definitions, but it's where AIX defines conventional socket
ioctl numbers.

Discussion: https://postgr.es/m/CA+hUKGKErNfhmvb_H0UprEmp4LPzGN06yR2_0tYikjzB-2ECMw@mail.gmail.com

configure

@@ -13761,7 +13761,7 @@ $as_echo "#define HAVE_STDBOOL_H 1" >>confdefs.h
 fi
 
 
-for ac_header in atomic.h copyfile.h execinfo.h getopt.h ifaddrs.h langinfo.h mbarrier.h net/if.h netinet/tcp.h sys/epoll.h sys/event.h sys/personality.h sys/prctl.h sys/procctl.h sys/signalfd.h sys/sockio.h sys/ucred.h termios.h ucred.h
+for ac_header in atomic.h copyfile.h execinfo.h getopt.h ifaddrs.h langinfo.h mbarrier.h netinet/tcp.h sys/epoll.h sys/event.h sys/personality.h sys/prctl.h sys/procctl.h sys/signalfd.h sys/sockio.h sys/ucred.h termios.h ucred.h
 do :
   as_ac_Header=`$as_echo "ac_cv_header_$ac_header" | $as_tr_sh`
 ac_fn_c_check_header_mongrel "$LINENO" "$ac_header" "$as_ac_Header" "$ac_includes_default"

configure.ac

@@ -1450,7 +1450,6 @@ AC_CHECK_HEADERS(m4_normalize([
 	ifaddrs.h
 	langinfo.h
 	mbarrier.h
-	net/if.h
 	netinet/tcp.h
 	sys/epoll.h
 	sys/event.h

src/backend/libpq/ifaddr.c

@@ -323,10 +323,7 @@ pg_foreach_ifaddr(PgIfAddrCallback callback, void *cb_data)
 #else							/* !HAVE_GETIFADDRS && !WIN32 */
 
 #include <sys/ioctl.h>
-
-#ifdef HAVE_NET_IF_H
 #include <net/if.h>
-#endif
 
 #ifdef HAVE_SYS_SOCKIO_H
 #include <sys/sockio.h>

src/include/pg_config.h.in

@@ -325,9 +325,6 @@
 /* Define to 1 if you have the <netinet/tcp.h> header file. */
 #undef HAVE_NETINET_TCP_H
 
-/* Define to 1 if you have the <net/if.h> header file. */
-#undef HAVE_NET_IF_H
-
 /* Define to 1 if you have the `OPENSSL_init_ssl' function. */
 #undef HAVE_OPENSSL_INIT_SSL
 

src/tools/msvc/Solution.pm

@@ -305,7 +305,6 @@ sub GenerateFiles
 		HAVE_MEMSET_S               => undef,
 		HAVE_MKDTEMP                => undef,
 		HAVE_NETINET_TCP_H          => undef,
-		HAVE_NET_IF_H               => undef,
 		HAVE_OPENSSL_INIT_SSL       => undef,
 		HAVE_OSSP_UUID_H            => undef,
 		HAVE_PAM_PAM_APPL_H         => undef,