mkmf.rb: remove unnecessary variables
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51757 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: remove unnecessary variables
top_srcdir is defined above, builddir is not used.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51757 b2dd03c8-39d4-4d8f-98ff-823fe69b080e