Rename configure.in to configure.ac (Bug#11603).
The name 'configure.in' has been obsolescent for quite some time,
and the next release of Autoconf will generate warnings for it.
+ See commit 'v2.69-4-g560f16b' of 2012-05-06, "general: deprecate
+ 'configure.in' as autoconf input" in the Autoconf git repository.
* configure.ac: Rename from configure.in.
* INSTALL, INSTALL.BZR, README, make-dist:
* Makefile.in (AUTOCONF_INPUTS):