This change fixes test_problems test on FreeBSD. Without that /usr/include isn't added to the default include paths and many system headers become missing.
I do understand that this is probably wrong place to make such a change, but after some research it turns out that Clang devs didn't put the logic for properly setting
default include paths into libclang. It is present in clang itself, but not libclang.
Relevant thread in another libclang consumer: https://github.com/Valloric/YouCompleteMe/issues/2330
So, what would be the proper fix for that?