automake: account for perl hash order randomization

automake: account for perl hash order randomization

Try to explicitly order the keys of some perl hashes when looping
on them to do sanity/correctness checks and possibly display warning
messages; this should ensure a more reproducible output.  Not really
a big deal, but I prefer to keep the order of such output reproducible
if possible.

Issue revealed by spurious testsuite failures with perl 5.18, as
reported in automake bug#14891.  See also:
<http://search.cpan.org/dist/perl-5.18.0/pod/perldelta.pod#Hash_randomization>
<http://onionstand.blogspot.ie/2012/12/are-you-relying-on-hash-keys-being.html>

* lib/Automake/Variable.pm (variables): Explicitly order the values of
the returned Automake::Variable instances.
(variables_dump): Simplify, using the knowledge that 'variables()' now
sorts its output.
* t/preproc-errmsg.sh: Adjust.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>