repo: don't commit generated files in the git repository anymore

repo: don't commit generated files in the git repository anymore

It has been quite some time since autoconf and libtool have stopped
committing the generated autotools files in their git repositories,
with no significant ill effects we're aware of.  It's true that the
autoconf bootstrap process has now the minor annoyance that a
pre-installed autoconf is required to complete it; but luckily
automake will not have a similar annoyance, since our bootstrap
script take care, through some hoops, to use the very automake and
aclocal versions from the current git checkout to generate the
required aclocal.m4 and Makefile.in files.  In fact, this has been
a necessity also in the past, because automake has been known to
use in its own build system new development features that hadn't
been present in any previously released automake distribution.

* aclocal.m4: Delete.
* configure: Likewise.
* Makefile.in: Likewise.
* doc/Makefile.in: Likewise.
* lib/Automake/Makefile.in: Likewise.
* lib/Automake/tests/Makefile.in: Likewise.
* lib/Makefile.in: Likewise.
* lib/am/Makefile.in: Likewise.
* m4/Makefile.in: Likewise.
* tests/Makefile.in: Likewise.
* .gitignore: Ignore configure, aclocal.m4, and all the
Makefile.in files.
* configure.ac (AC_PREREQ): New macro call, to require the
latest autoconf (2.68 for the moment).