Trust a few system headers to stand on their own

At some point in the past some headers (net/if.h on some BSDs in 2009,
netinet/tcp.h on IRIX in 2000, sys/ucred.h in 2013 on then older openbsd),
only compiled if other heades were included first, complicating configure
tests. More recent tests indicate that that's not required anymore.

Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20220807012914.ydz73yte6j3coulo@awork3.anarazel.de

configure

@@ -13874,7 +13874,7 @@ $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 sys/epoll.h sys/event.h sys/ipc.h sys/personality.h sys/prctl.h sys/procctl.h sys/resource.h sys/select.h sys/sem.h sys/shm.h sys/signalfd.h sys/sockio.h sys/uio.h sys/un.h termios.h ucred.h
+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/ipc.h sys/personality.h sys/prctl.h sys/procctl.h sys/resource.h sys/select.h sys/sem.h sys/shm.h sys/signalfd.h sys/sockio.h sys/ucred.h sys/uio.h sys/un.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"
@@ -13888,60 +13888,6 @@ fi
 done
 
 
-# On BSD, test for net/if.h will fail unless sys/socket.h
-# is included first.
-for ac_header in net/if.h
-do :
-  ac_fn_c_check_header_compile "$LINENO" "net/if.h" "ac_cv_header_net_if_h" "$ac_includes_default
-#include <sys/socket.h>
-
-"
-if test "x$ac_cv_header_net_if_h" = xyes; then :
-  cat >>confdefs.h <<_ACEOF
-#define HAVE_NET_IF_H 1
-_ACEOF
-
-fi
-
-done
-
-
-# On OpenBSD, test for sys/ucred.h will fail unless sys/param.h
-# is included first.
-for ac_header in sys/ucred.h
-do :
-  ac_fn_c_check_header_compile "$LINENO" "sys/ucred.h" "ac_cv_header_sys_ucred_h" "$ac_includes_default
-#include <sys/param.h>
-
-"
-if test "x$ac_cv_header_sys_ucred_h" = xyes; then :
-  cat >>confdefs.h <<_ACEOF
-#define HAVE_SYS_UCRED_H 1
-_ACEOF
-
-fi
-
-done
-
-
-# At least on IRIX, test for netinet/tcp.h will fail unless
-# netinet/in.h is included first.
-for ac_header in netinet/tcp.h
-do :
-  ac_fn_c_check_header_compile "$LINENO" "netinet/tcp.h" "ac_cv_header_netinet_tcp_h" "$ac_includes_default
-#include <netinet/in.h>
-
-"
-if test "x$ac_cv_header_netinet_tcp_h" = xyes; then :
-  cat >>confdefs.h <<_ACEOF
-#define HAVE_NETINET_TCP_H 1
-_ACEOF
-
-fi
-
-done
-
-
 if expr x"$pgac_cv_check_readline" : 'x-lreadline' >/dev/null ; then
   for ac_header in readline/readline.h
 do :

configure.ac

@@ -1452,6 +1452,8 @@ AC_CHECK_HEADERS(m4_normalize([
 	ifaddrs.h
 	langinfo.h
 	mbarrier.h
+	net/if.h
+	netinet/tcp.h
 	sys/epoll.h
 	sys/event.h
 	sys/ipc.h
@@ -1464,33 +1466,13 @@ AC_CHECK_HEADERS(m4_normalize([
 	sys/shm.h
 	sys/signalfd.h
 	sys/sockio.h
+	sys/ucred.h
 	sys/uio.h
 	sys/un.h
 	termios.h
 	ucred.h
 ]))
 
-# On BSD, test for net/if.h will fail unless sys/socket.h
-# is included first.
-AC_CHECK_HEADERS(net/if.h, [], [],
-[AC_INCLUDES_DEFAULT
-#include <sys/socket.h>
-])
-
-# On OpenBSD, test for sys/ucred.h will fail unless sys/param.h
-# is included first.
-AC_CHECK_HEADERS(sys/ucred.h, [], [],
-[AC_INCLUDES_DEFAULT
-#include <sys/param.h>
-])
-
-# At least on IRIX, test for netinet/tcp.h will fail unless
-# netinet/in.h is included first.
-AC_CHECK_HEADERS(netinet/tcp.h, [], [],
-[AC_INCLUDES_DEFAULT
-#include <netinet/in.h>
-])
-
 if expr x"$pgac_cv_check_readline" : 'x-lreadline' >/dev/null ; then
   AC_CHECK_HEADERS(readline/readline.h, [],
         [AC_CHECK_HEADERS(readline.h, [],