mkmf.rb: prefix install_dirs only with DESTDIR
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39841 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mkmf.rb: prefix install_dirs only with DESTDIR
directories to install only unless bundled extension libraries.
[ruby-core:53502] [Bug #8115]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39841 b2dd03c8-39d4-4d8f-98ff-823fe69b080e