mkmf.rb: revert DESTDIR prefix
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41756 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: revert DESTDIR prefix
it is fixed by r41648. [ruby-core:55760] [Bug #8115]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41756 b2dd03c8-39d4-4d8f-98ff-823fe69b080e