OS X: Use -iframework with AppleClang only on version >= 4.2

Since commit v3.1.0-rc1~564^2 (OS X: Use -iframework for system
framework directories, 2014-05-05) we test the version of Clang to see
if it supports -iframework.  Fix the version test used for AppleClang
since it uses a different version scheme than upstream Clang.