* configure.in: check for libunwind.h, which is not available in
	  very old OS X SDK.  [ruby-core:71080] [Bug #11591]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@52354 b2dd03c8-39d4-4d8f-98ff-823fe69b080e