configure.in: declare a requirement for at least autoconf 2.50

configure.in: declare a requirement for at least autoconf 2.50

On Debian/Ubuntu systems, if autoconf version 2.13 is installed,
autoconf will try to automatically figure out whether autoconf 2.13 or
something more modern is required (since the autoconf maintainers,
curses be upon them, didn't bother to maintain compatibility between
autoconf 2.13 and 2.50).  Unfortunately, the hueristics aren't
perfect, and although the configure.in file looks superficially like
it will be compatible with autoconf 2.13, it isn't.  You will end up
with a number of very subtle compilation failures if you use autoconf
2.13.

So declare a requirement for autoconf 2.50 using AC_PREREQ(2.50).

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>