Add Gnulib 'gethostname' module, needed for MinGW.

This is again from Gnulib v0.1-5703-g356a414e8c.

* m4/gnulib-cache.m4: Add 'gethostname'.

m4/gethostname.m4

@@ -0,0 +1,107 @@
+# gethostname.m4 serial 15
+dnl Copyright (C) 2002, 2008-2023 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+# Ensure
+# - the gethostname() function,
+# - the HOST_NAME_MAX macro in <limits.h>.
+AC_DEFUN([gl_FUNC_GETHOSTNAME],
+[
+  AC_REQUIRE([gl_UNISTD_H_DEFAULTS])
+  gl_PREREQ_SYS_H_WINSOCK2
+
+  dnl Where is gethostname() defined?
+  dnl - On native Windows, it is in ws2_32.dll.
+  dnl - Otherwise it is in libc.
+  GETHOSTNAME_LIB=
+  AC_CHECK_FUNCS([gethostname], , [
+    AC_CACHE_CHECK([for gethostname in winsock2.h and -lws2_32],
+      [gl_cv_w32_gethostname],
+      [gl_cv_w32_gethostname=no
+       gl_save_LIBS="$LIBS"
+       LIBS="$LIBS -lws2_32"
+       AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+#ifdef HAVE_WINSOCK2_H
+#include <winsock2.h>
+#endif
+#include <stddef.h>
+]], [[gethostname(NULL, 0);]])], [gl_cv_w32_gethostname=yes])
+       LIBS="$gl_save_LIBS"
+      ])
+    if test "$gl_cv_w32_gethostname" = "yes"; then
+      GETHOSTNAME_LIB="-lws2_32"
+    fi
+  ])
+  AC_SUBST([GETHOSTNAME_LIB])
+
+  if test "$ac_cv_func_gethostname" = no; then
+    HAVE_GETHOSTNAME=0
+  fi
+
+  gl_PREREQ_HOST_NAME_MAX
+])
+
+# Provide HOST_NAME_MAX when <limits.h> lacks it.
+AC_DEFUN([gl_PREREQ_HOST_NAME_MAX], [
+  dnl - On most Unix systems, use MAXHOSTNAMELEN from <sys/param.h> instead.
+  dnl - On Solaris, Cygwin, BeOS, use MAXHOSTNAMELEN from <netdb.h> instead.
+  dnl - On mingw, use 256, because
+  dnl   <https://docs.microsoft.com/en-us/windows/desktop/api/winsock/nf-winsock-gethostname> says:
+  dnl   "if a buffer of 256 bytes is passed in the name parameter and
+  dnl    the namelen parameter is set to 256, the buffer size will always
+  dnl    be adequate."
+  dnl With this, there is no need to use sysconf (_SC_HOST_NAME_MAX), which
+  dnl is not a compile-time constant.
+  dnl We cannot override <limits.h> using the usual technique, because
+  dnl gl_CHECK_NEXT_HEADERS does not work for <limits.h>. Therefore retrieve
+  dnl the value of HOST_NAME_MAX at configure time.
+  AC_CHECK_HEADERS_ONCE([sys/param.h])
+  AC_CHECK_HEADERS_ONCE([sys/socket.h])
+  AC_CHECK_HEADERS_ONCE([netdb.h])
+  AC_CACHE_CHECK([for HOST_NAME_MAX], [gl_cv_decl_HOST_NAME_MAX], [
+    gl_cv_decl_HOST_NAME_MAX=
+    AC_EGREP_CPP([lucky], [
+#include <limits.h>
+#ifdef HOST_NAME_MAX
+lucky
+#endif
+      ], [gl_cv_decl_HOST_NAME_MAX=yes])
+    if test -z "$gl_cv_decl_HOST_NAME_MAX"; then
+      dnl It's not defined in <limits.h>. Substitute it.
+      if test "$gl_cv_w32_gethostname" = yes; then
+        dnl mingw.
+        gl_cv_decl_HOST_NAME_MAX=256
+      else
+        AC_COMPUTE_INT([gl_cv_decl_HOST_NAME_MAX], [MAXHOSTNAMELEN], [
+#include <sys/types.h>
+#if HAVE_SYS_PARAM_H
+# include <sys/param.h>
+#endif
+#if HAVE_SYS_SOCKET_H
+# include <sys/socket.h>
+#endif
+#if HAVE_NETDB_H
+# include <netdb.h>
+#endif
+],
+          [dnl The system does not define MAXHOSTNAMELEN in any of the common
+           dnl headers. Use a safe fallback.
+           gl_cv_decl_HOST_NAME_MAX=256
+          ])
+      fi
+    fi
+  ])
+  if test "$gl_cv_decl_HOST_NAME_MAX" != yes; then
+    AC_DEFINE_UNQUOTED([HOST_NAME_MAX], [$gl_cv_decl_HOST_NAME_MAX],
+      [Define HOST_NAME_MAX when <limits.h> does not define it.])
+  fi
+])
+
+# Prerequisites of lib/gethostname.c.
+AC_DEFUN([gl_PREREQ_GETHOSTNAME], [
+  if test "$gl_cv_w32_gethostname" != "yes"; then
+    AC_CHECK_FUNCS([uname])
+  fi
+])

m4/gnulib-cache.m4

@@ -78,6 +78,7 @@
 #  func \
 #  gendocs \
 #  getaddrinfo \
+#  gethostname \
 #  getlogin \
 #  getpeername \
 #  getsockname \
@@ -183,6 +184,7 @@ gl_MODULES([
   func
   gendocs
   getaddrinfo
+  gethostname
   getlogin
   getpeername
   getsockname

m4/gnulib-comp.m4

@@ -109,6 +109,7 @@ AC_DEFUN([gl_EARLY],
   # Code from module gendocs:
   # Code from module getaddrinfo:
   # Code from module getdtablesize:
+  # Code from module gethostname:
   # Code from module getlogin:
   # Code from module getpeername:
   # Code from module getrandom:
@@ -422,6 +423,12 @@ AC_DEFUN([gl_INIT],
   gl_CONDITIONAL([GL_COND_OBJ_GAI_STRERROR],
                  [test $HAVE_DECL_GAI_STRERROR = 0 || test $REPLACE_GAI_STRERROR = 1])
   gl_NETDB_MODULE_INDICATOR([getaddrinfo])
+  gl_FUNC_GETHOSTNAME
+  gl_CONDITIONAL([GL_COND_OBJ_GETHOSTNAME], [test $HAVE_GETHOSTNAME = 0])
+  AM_COND_IF([GL_COND_OBJ_GETHOSTNAME], [
+    gl_PREREQ_GETHOSTNAME
+  ])
+  gl_UNISTD_MODULE_INDICATOR([gethostname])
   gl_FUNC_GETLOGIN
   gl_CONDITIONAL([GL_COND_OBJ_GETLOGIN], [test $HAVE_GETLOGIN = 0])
   gl_UNISTD_MODULE_INDICATOR([getlogin])
@@ -1664,6 +1671,9 @@ AC_SUBST([LTALLOCA])
   if test $HAVE_GETADDRINFO = 0 || test $REPLACE_GETADDRINFO = 1; then
     func_gl_gnulib_m4code_sockets
   fi
+  if test $HAVE_GETHOSTNAME = 0; then
+    func_gl_gnulib_m4code_sockets
+  fi
   if test $HAVE_INET_NTOP = 0 || test $REPLACE_INET_NTOP = 1; then
     func_gl_gnulib_m4code_netinet_in
   fi
@@ -2113,6 +2123,7 @@ AC_DEFUN([gl_FILE_LIST], [
   lib/gai_strerror.c
   lib/getaddrinfo.c
   lib/getdtablesize.c
+  lib/gethostname.c
   lib/getlogin.c
   lib/getpeername.c
   lib/getrandom.c
@@ -2381,6 +2392,7 @@ AC_DEFUN([gl_FILE_LIST], [
   m4/func.m4
   m4/getaddrinfo.m4
   m4/getdtablesize.m4
+  m4/gethostname.m4
   m4/getlogin.m4
   m4/getrandom.m4
   m4/gnulib-common.m4