merge revision(s) 44019: [Backport #9213]

	* lib/mkmf.rb (configuration): strip destdir part from prefix to get
	  rid of duplication.  a patch by arton at [ruby-core:58859].
	  [ruby-core:58856] [Bug #9213]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@44367 b2dd03c8-39d4-4d8f-98ff-823fe69b080e