Move gnulib to top level

This patch moves the gdb/gnulib subdirectory to the top level.

It adjusts the top-level build system to build gnulib when necessary,
and changes gdb to use this.  However, gdbserver still builds its own
copy of gnulib, just from the new source location.

A small hack was needed to ensure that gnulib is only built when gdb
is enabled.  The Makefile only provides an ordering -- the directory
must be mentioned in configdirs to actually be compiled at all.

Most of the patch is just a "git mv" of gnulib, though a few minor
path adjustments were needed in some files there.

Tested by the buildbot.

ChangeLog
2019-06-14  Tom Tromey  <tom@tromey.com>

	* MAINTAINERS: Add gnulib.
	* gnulib: New directory, move from gdb/gnulib.
	* configure.ac (host_libs): Add gnulib.
	* configure: Rebuild.
	* Makefile.def (host_modules, dependencies): Add gnulib.
	* Makefile.in: Rebuild.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tom@tromey.com>

	* gnulib: Move directory to top-level.
	* configure.ac: Don't configure gnulib.
	* configure: Rebuild.
	* common/common-defs.h: Use new path to gnulib.
	* Makefile.in (GNULIB_BUILDDIR): Now ../gnulib.
	(GNULIB_H): Remove.
	(INCGNU): Look in new gnulib location.
	(HFILES_NO_SRCDIR): Remove gnulib files.
	(SUBDIR, REQUIRED_SUBDIRS): Remove gnulib.
	(generated_files): Remove GNULIB_H.
	($(LIBGNU), all-lib): Remove targets.
	(distclean): Don't mention GNULIB_BUILDDIR.
	($(GNULIB_BUILDDIR)/Makefile): Remove target.

gdb/gdbserver/ChangeLog
2019-06-14  Tom Tromey  <tom@tromey.com>

	* configure.ac: Use new path to gnulib.
	* configure: Rebuild.
	* Makefile.in (INCGNU, $(GNULIB_BUILDDIR)/Makefile): Use new path
	to gnulib.

gnulib/ChangeLog
2019-06-14  Tom Tromey  <tom@tromey.com>

	* update-gnulib.sh: Adjust paths.
	* Makefile.in: Adjust paths.
	* configure.ac: Adjust paths.  Use ACX_LARGEFILE.
	* configure: Rebuild.

gnulib/import/readlink.c

@@ -0,0 +1,74 @@
+/* Stub for readlink().
+   Copyright (C) 2003-2007, 2009-2016 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <unistd.h>
+
+#include <errno.h>
+#include <string.h>
+#include <sys/stat.h>
+
+#if !HAVE_READLINK
+
+/* readlink() substitute for systems that don't have a readlink() function,
+   such as DJGPP 2.03 and mingw32.  */
+
+ssize_t
+readlink (const char *name, char *buf _GL_UNUSED,
+          size_t bufsize _GL_UNUSED)
+{
+  struct stat statbuf;
+
+  /* In general we should use lstat() here, not stat().  But on platforms
+     without symbolic links, lstat() - if it exists - would be equivalent to
+     stat(), therefore we can use stat().  This saves us a configure check.  */
+  if (stat (name, &statbuf) >= 0)
+    errno = EINVAL;
+  return -1;
+}
+
+#else /* HAVE_READLINK */
+
+# undef readlink
+
+/* readlink() wrapper that uses correct types, for systems like cygwin
+   1.5.x where readlink returns int, and which rejects trailing slash,
+   for Solaris 9.  */
+
+ssize_t
+rpl_readlink (const char *name, char *buf, size_t bufsize)
+{
+# if READLINK_TRAILING_SLASH_BUG
+  size_t len = strlen (name);
+  if (len && name[len - 1] == '/')
+    {
+      /* Even if name without the slash is a symlink to a directory,
+         both lstat() and stat() must resolve the trailing slash to
+         the directory rather than the symlink.  We can therefore
+         safely use stat() to distinguish between EINVAL and
+         ENOTDIR/ENOENT, avoiding extra overhead of rpl_lstat().  */
+      struct stat st;
+      if (stat (name, &st) == 0)
+        errno = EINVAL;
+      return -1;
+    }
+# endif /* READLINK_TRAILING_SLASH_BUG */
+  return readlink (name, buf, bufsize);
+}
+
+#endif /* HAVE_READLINK */