What is needed for these external typedefs to work in C++?

I'm trying to compile QT 5.11.1 on Linux. GCC is 5.3.0. C++ type option is -std=gnu++14.

In some C++ modules, there are references to external typedefs of structs and enums, defined in .h files, which do not work with diagnostics: "was not declared in this scope".

The (examples of) definitions not working so far are:

typedef struct hb_variation_t {
  hb_tag_t tag;
  float    value;
} hb_variation_t;

--not accessible from within named namespace "blink" and

typedef enum { /*< flags >*/
  HB_GLYPH_FLAG_UNSAFE_TO_BREAK         = 0x00000001,
  HB_GLYPH_FLAG_DEFINED                 = 0x00000001 /* OR of all defined flags
} hb_glyph_flags_t;

--not accessible from within anonymous namespace.

Are there any compiler options or preprocessor directives?