extmk.rb: rename variables

* ext/extmk.rb: rename working variables as more descriptive so no
  shadowing local variable warnings, and remove unused variables.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39336 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
extmk.rb: extract SUBMAKEOPTS

* ext/extmk.rb (SUBMAKEOPTS): extract common sub make options.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39337 b2dd03c8-39d4-4d8f-98ff-823fe69b080e