Wall should be -Wall
[MakeMaker] 5.020 no longer has a default value for eumm_version
remove redundant code to calculate test/build prereqs

as done for [MakeMaker], rjbs/Dist-Zilla@c3197585, Graham Knop
skip passing empty hashrefs, arrayrefs to WriteMakefileArgs

This was resulting in ugly prereqs in META.json like:

    "prereqs" : {
       "build" : {
          "requires" : {}
       },
       "test" : {
          "requires" : {}
       },
       ...
    },

As done on rjbs/Dist-Zilla@84c1b6a0, Karen Etheridge
Dist-Zilla-Plugin-MakeMaker-Awesome-0.22

    - fix for Dist::Zilla::Plugin::MakeMaker 5.020 dropping a default required
      version for ExtUtils::MakeMaker
    - now omitting BUILD_REQUIRES, TEST_REQUIRES in Makefile.PL when empty