Move it to a separate section, since hb-common says include hb.h, while
we want to include hb-features.h here.
This still does not fix generating documentation of undefined macros
(e.g. HB_HAS_GDI since we build docs on Linux).
This header has defines for all the optional
dependendencies that come with their own Harfbuzz
headers, so you can do:
#include <hb-features.h>
#ifdef HB_HAS_DIRECTWRITE
#include <hb-directwrite.h>
#endif