merge revision(s) 47248: [Backport #10161]

	* common.mk (Doxyfile): revert r43888, not to require preinstalled
	  ruby.  [ruby-core:64488] [Bug #10161]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@47546 b2dd03c8-39d4-4d8f-98ff-823fe69b080e