Remove configure probe for sys/select.h.

<sys/select.h> is in SUSv3 and every targeted Unix system has it.
Provide an empty header in src/include/port/win32 so that we can
include it unguarded even on Windows.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKG%2BL_3brvh%3D8e0BW_VfX9h7MtwgN%3DnFHP5o7X2oZucY9dg%40mail.gmail.com

src/backend/libpq/auth.c

@@ -16,13 +16,11 @@
 #include "postgres.h"
 
 #include <sys/param.h>
+#include <sys/select.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <pwd.h>
 #include <unistd.h>
-#ifdef HAVE_SYS_SELECT_H
-#include <sys/select.h>
-#endif
 
 #include "commands/user.h"
 #include "common/ip.h"

src/backend/postmaster/postmaster.c

@@ -70,6 +70,7 @@
 #include <time.h>
 #include <sys/wait.h>
 #include <ctype.h>
+#include <sys/select.h>
 #include <sys/stat.h>
 #include <sys/socket.h>
 #include <fcntl.h>
@@ -77,10 +78,6 @@
 #include <netdb.h>
 #include <limits.h>
 
-#ifdef HAVE_SYS_SELECT_H
-#include <sys/select.h>
-#endif
-
 #ifdef USE_BONJOUR
 #include <dns_sd.h>
 #endif

src/backend/tcop/postgres.c

@@ -23,10 +23,8 @@
 #include <limits.h>
 #include <signal.h>
 #include <unistd.h>
-#include <sys/socket.h>
-#ifdef HAVE_SYS_SELECT_H
 #include <sys/select.h>
-#endif
+#include <sys/socket.h>
 #ifdef HAVE_SYS_RESOURCE_H
 #include <sys/time.h>
 #include <sys/resource.h>