And what exact configuration option should I check?

In the Makefile.PL, I would:

if($Config::Config{ivsize} < 8) {
  print "Bailing out - 64-bit integers (IV) are required\n";
  exit 0;
}
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As pryrt correctly suggested, this will result in no cpan-tester feedback being sent to you. Nor will it be registered anywhere as a "FAIL".
There's actually no need to explicitly use Config; (though you can, if you want) because ExtUtils::MakeMaker has already loaded it.

Cheers,
Rob