* ext/extmk.rb: Re-generate extmk.mk and dummy makefiles only if
  really required.
  This fixes a problem to run multiple test-all concurrently as:
  make test-all & make test-all & make test-all & ...


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45591 b2dd03c8-39d4-4d8f-98ff-823fe69b080e