mkmf.rb: prefix install_dirs only with DESTDIR

* lib/mkmf.rb (install_dirs, with_destdir): prefix 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
mkmf.rb: format message

* lib/mkmf.rb (find_library): fix to format message.
  [ruby-core:53568] [Bug #8130]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39842 b2dd03c8-39d4-4d8f-98ff-823fe69b080e