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.