Add Gnulib 'gethostname' module, needed for MinGW.

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

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

lib/Makefile.am

@@ -73,6 +73,7 @@
 #  func \
 #  gendocs \
 #  getaddrinfo \
+#  gethostname \
 #  getlogin \
 #  getpeername \
 #  getsockname \
@@ -179,6 +180,7 @@ libgnu_la_LDFLAGS += $(DUPLOCALE_LIB)
 libgnu_la_LDFLAGS += $(FLOOR_LIBM)
 libgnu_la_LDFLAGS += $(FREXP_LIBM)
 libgnu_la_LDFLAGS += $(GETADDRINFO_LIB)
+libgnu_la_LDFLAGS += $(GETHOSTNAME_LIB)
 libgnu_la_LDFLAGS += $(GETLOGIN_LIB)
 libgnu_la_LDFLAGS += $(GETRANDOM_LIB)
 libgnu_la_LDFLAGS += $(HARD_LOCALE_LIB)
@@ -856,6 +858,16 @@ endif
 endif
 ## end   gnulib module getdtablesize
 
+## begin gnulib module gethostname
+
+if GL_COND_OBJ_GETHOSTNAME
+libgnu_la_SOURCES += gethostname.c
+endif
+
+EXTRA_DIST += w32sock.h
+
+## end   gnulib module gethostname
+
 ## begin gnulib module getlogin
 
 if GL_COND_OBJ_GETLOGIN

lib/gethostname.c

@@ -0,0 +1,104 @@
+/* gethostname emulation for SysV and POSIX.1.
+
+   Copyright (C) 1992, 2003, 2006, 2008-2023 Free Software Foundation, Inc.
+
+   This file is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   This file is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* David MacKenzie <djm@gnu.ai.mit.edu>
+   Windows port by Simon Josefsson <simon@josefsson.org> */
+
+#include <config.h>
+
+#if !(defined _WIN32 && !defined __CYGWIN__)
+/* Unix API.  */
+
+/* Specification.  */
+#include <unistd.h>
+
+#ifdef HAVE_UNAME
+# include <sys/utsname.h>
+#endif
+
+#include <string.h>
+
+/* Put up to LEN chars of the host name into NAME.
+   Null terminate it if the name is shorter than LEN.
+   Return 0 if ok, -1 if error.  */
+
+#include <stddef.h>
+
+int
+gethostname (char *name, size_t len)
+{
+#ifdef HAVE_UNAME
+  struct utsname uts;
+
+  if (uname (&uts) == -1)
+    return -1;
+  if (len > sizeof (uts.nodename))
+    {
+      /* More space than we need is available.  */
+      name[sizeof (uts.nodename)] = '\0';
+      len = sizeof (uts.nodename);
+    }
+  strncpy (name, uts.nodename, len);
+#else
+  strcpy (name, "");            /* Hardcode your system name if you want.  */
+#endif
+  return 0;
+}
+
+#else
+/* Native Windows API.  Which primitive to choose?
+   - gethostname() requires linking with -lws2_32.
+   - GetComputerName() does not return the right kind of hostname.
+   - GetComputerNameEx(ComputerNameDnsHostname,...) returns the right hostname,
+     but it is hard to use portably:
+       - It requires defining _WIN32_WINNT to at least 0x0500.
+       - With mingw, it also requires
+         "#define GetComputerNameEx GetComputerNameExA".
+       - With older versions of mingw, none of the declarations are present at
+         all, not even of the enum value ComputerNameDnsHostname.
+   So we use gethostname().  Linking with -lws2_32 is the least evil.  */
+
+#define WIN32_LEAN_AND_MEAN
+/* Get winsock2.h. */
+#include <unistd.h>
+
+/* Get INT_MAX.  */
+#include <limits.h>
+
+/* Get set_winsock_errno. */
+#include "w32sock.h"
+
+#include "sockets.h"
+
+#undef gethostname
+
+int
+rpl_gethostname (char *name, size_t len)
+{
+  int r;
+
+  if (len > INT_MAX)
+    len = INT_MAX;
+  gl_sockets_startup (SOCKETS_1_1);
+  r = gethostname (name, (int) len);
+  if (r < 0)
+    set_winsock_errno ();
+
+  return r;
+}
+
+#endif