In line with Gnulib, we now require Autoconf 2.63 and Automake 1.11.

* bootstrap: Sync to Gnulib d271f868a8df9bbec29049d01e056481b7a1a263.
* bootstrap.conf (GNULIB_REVISION): Set to
d271f868a8df9bbec29049d01e056481b7a1a263.
(buildreq): Bump required autoconf version to 2.63.
* configure.ac (AM_INIT_AUTOMAKE): Bump to 1.11.
(AM_SILENT_RULES, AM_PROG_AR): Remove conditionals, which were there for
Automake 1.10 support.
(AC_PREREQ): Bump to 2.63.
* NEWS: Document this.