Set mode for GNU Makefile.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41491 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Remove an unnecessary mode comment.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41492 b2dd03c8-39d4-4d8f-98ff-823fe69b080e