Add support for -iframework flag

Just like `-I` flag has its `-isystem` counterpart which marks an include directory
as a system directory and prevents unwanted warnings, on Apple systems there is
`-iframework` -- a system directory replacement for `-F`. This commits adds support
for this flag using existing CMake facilities (`include_directories(SYSTEM)`).

This has been bothering for so long, requiring all sorts of nasty hacks,
that I finally decided to try and fix it. This is my first time contributing to CMake, 
but the patch is fairly trivial, so I hope I haven't really had chance to make some
stupid mistake or break any rules.