Not all C++11 features are created equal when it comes to support...

lvalue iterators must declare __item_type__ as a reference.

Ugly, but does the job.

Still not satisfied with how I can enforce iterators only, but
seems to work for now.

Not C++11.

../../third_party/harfbuzz-ng/src/src/hb-map.hh(56,45):  warning: comparison of integers of different signs: 'const hb_codepoint_t' (aka 'const unsigned int') and 'hb_map_t::(anonymous enum at ../../third_party/harfbuzz-ng/src/src/hb-map.hh:169:3)' [-Wsign-compare]
    bool is_unused () const    { return key == INVALID; }

warning: '_WIN64' is not defined, evaluates to 0 [-Wundef]

Fixes #1548 hopefully

Fixes https://github.com/harfbuzz/harfbuzz/issues/1549