merge revision(s) 36045: [Backport #7637]

	* lib/mkmf.rb: add dummy clean-static target to prevent errors for the
	  case real clean-static target doesn't exist.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@38780 b2dd03c8-39d4-4d8f-98ff-823fe69b080e