Stop release.pl --setver failing if Makefile exists.

This should have been part of commit ea0ab1c82; it's part of the
general revamp that we regenerate the autoconf files ourselves in a
clean directory - so we don't depend on them being present, but we
also don't depend on them being _absent_ either.

But when I made that commit my immediate priority was to get --setver
to work from a completely clean checkout, not from one already
littered with cruft, so I didn't check quite as carefully that my
changes fixed the problem in the latter case too :-)

release.pl

@@ -29,7 +29,6 @@ if ($setver) {
     0 == system "git", "diff-index", "--quiet", "--cached", "HEAD"
         or die "index is dirty";
     0 == system "git", "diff-files", "--quiet" or die "working tree is dirty";
-    -f "Makefile" and die "run 'make distclean' first";
     my $builddir = tempdir(DIR => ".", CLEANUP => 1);
     0 == system "git archive --format=tar HEAD | ( cd $builddir && tar xf - )"
         or die;