Autoconf 2.51

[Akim Demaille]

I'm willing to release 2.51 more or less this weekend.  There are
still some open bugs, some of which I'm going to kill tomorrow (only
the trivial ones).  Others require more adventurous patches, and are
delayed to 2.52 or later.

If you know regressions between 2.50 and 2.50a, please let us know.

If you know _serious_ (i.e., _many_ packages are concerned) bugs which
can justify a delayed release of 2.51, please speak up.  But before
that, make sure you do consider 2.50a is not good enough _compared to
2.50_.

        Akim