Support for configure.ac

[Akim Demaille]

Here is my proposal.  I've tested it by s/configure.in/configure.ac/g
the test suite, and it works.  I have not tested with a real package
yet though.

I was not sure where you would have preferred that automake detects
configure.ac vs configure.in: I've done this in scan_autoconf_files
(former scan_configure), but maybe the top level would have been
better.

I really apologize for the length of the patch, but I don't have
enough experience to know a priori how to segment my patch by stages.
So this is the whole stuff.  It shouldn't be too hard to read though.

        Akim

automake-configure-ac.patch
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