merge revision(s) 39841: [Backport #8115]

	* 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/branches/ruby_2_0_0@41193 b2dd03c8-39d4-4d8f-98ff-823fe69b080e