autoupdate: replace obsolete macros in libiberty

Autoreconf-2.72 warns about obsolete macros. This patch aims at removing
the noise from a future upgrade to autoreconf-2.72 or later. This is in
no a way a complete patch allowing the upgrade to autoreconf-2.72.

- AC_GNU_SOURCE by AC_USE_SYSTEM_EXTENSIONS
  https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.72/
  autoconf.html#index-AC_005fGNU_005fSOURCE-1
- AC_CONFIG_HEADER by AC_CONFIG_HEADERS
  https://www.gnu.org/software/automake/manual/1.12.2/html_node/Obsolete-
  Macros.html#index-AM_005fCONFIG_005fHEADER

Those fixes were originally submitted in a patch series in binutils.
https://inbox.sourceware.org/binutils/878qthm6a0.fsf@gentoo.org/

libiberty/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Fix autoupdate warnings.

libiberty/configure

@@ -4413,7 +4413,6 @@ $as_echo "$ac_cv_safe_to_define___extensions__" >&6; }
   $as_echo "#define _TANDEM_SOURCE 1" >>confdefs.h
 
 
-
 # Check whether --enable-largefile was given.
 if test "${enable_largefile+set}" = set; then :
   enableval=$enable_largefile;

libiberty/configure.ac

@@ -172,7 +172,7 @@ AC_MSG_NOTICE([target_header_dir = $target_header_dir])
 
 GCC_NO_EXECUTABLES
 AC_PROG_CC
-AC_GNU_SOURCE
+AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
 AC_PROG_CPP_WERROR
 
@@ -205,7 +205,7 @@ dnl AM_PROG_LIBTOOL
 
 dnl When we start using automake:
 dnl AM_CONFIG_HEADER(config.h:config.in)
-AC_CONFIG_HEADER(config.h:config.in)
+AC_CONFIG_HEADERS([config.h:config.in])
 
 dnl When we start using automake:
 dnl AM_MAINTAINER_MODE