merge revision(s) 39842: [Backport #8130]

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


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40255 b2dd03c8-39d4-4d8f-98ff-823fe69b080e