build: require autoconf >= 2.62, and related simplifications

* configure.ac: Require 2.62, and remove obsolescent comments.
We are not imposing an new restriction in doing so, since we already
require Automake 1.11 or later, and that requires Autoconf 2.62.
Simplify code for symlinking of GNUmakefile in VPATH builds, in
accordance with the existing "TODO" comments.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>

configure.ac

@@ -16,8 +16,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-# We need AC_CONFIG_TESTDIR, AC_PROG_SED, AC_PROG_GREP.
-AC_PREREQ([2.60])
+AC_PREREQ([2.62])
 
 AC_INIT([GNU Autoconf],
 	m4_esyscmd([build-aux/git-version-gen .tarball-version]),
@@ -205,19 +204,14 @@ AC_PROG_MAKE_CASE_SENSITIVE
 ## Conclusion.  ##
 ## ------------ ##
 
-dnl Allow maintainer rules under GNU make even in VPATH builds.  This does
-dnl not work in autoconf 2.61 or earlier, but we don't want to require
-dnl unreleased autoconf during bootstrap, hence the version test.
-dnl TODO remove the version check once we depend on autoconf 2.62.
+dnl Allow maintainer rules under GNU make even in VPATH builds.
 dnl Meanwhile, we must use a shell variable so that we bypass automake's
 dnl attempts to remove the sole copy of GNUmakefile in a non-VPATH build
 dnl during 'make distclean'.
 dnl TODO avoid the shell variable once automake is fixed.
 GNUmakefile=GNUmakefile
-m4_if(m4_version_compare([2.61a.100],
-	m4_defn([m4_PACKAGE_VERSION])), [1], [],
-      [AC_CONFIG_LINKS([$GNUmakefile:$GNUmakefile], [],
-	[GNUmakefile=$GNUmakefile])])
+AC_CONFIG_LINKS([$GNUmakefile:$GNUmakefile], [],
+                [GNUmakefile=$GNUmakefile])
 
 AC_CONFIG_FILES([Makefile doc/Makefile
 		 lib/Makefile lib/Autom4te/Makefile lib/autoscan/Makefile