bytes.pm was introduced with 5.6. This means any code which has 'use
bytes' in it won't even compile on 5.5.X. Since bytes is a lexical
pragma and must be used at compile time we can't simply wrap it in
a BEGIN { eval 'use bytes' } block.
ExtUtils::MakeMaker::bytes is just a very thin wrapper around bytes
which works just like it when bytes.pm exists and everywhere else it
does nothing.