configure.{in -> ac} rename (commit 35eafcc71b) broke in-tree binutils building of gcc

[Jan Beulich]

Alan, gcc maintainers,

I was quite surprised for my gcc 4.9.3 build (using binutils 2.25 instead
of 2.24 as I had in use with 4.9.2) to fail in rather obscure ways. Quite
a bit of digging resulted in me finding that gcc/configure.ac looks for
configure.in in a number of binutils subtrees. Globally replacing
configure.in by configure.[ai][cn] appears to address this, but I'm not
sure whether that would be an acceptable change (there doesn't seem
to be a fix for this in gcc trunk either, which I originally expected I could
simply backport).

Thoughts?

Thanks, Jan