qdoc trips over std definitions on MacOS

While generating documentation for modules such as qtscxml or qtcharts, the qtdoc fails when it is facing things in std namespace. As a result it is unable to match functions to their documentation properly.

The std is the clearest symptom, but it may also be just a symptom of more fundamental thing about qdoc not seeing / finding header files appropriately.

Running with QDOC_QDEBUG=1 like this:

rm -fr * && cmake -GNinja ../qtscxml &&  QDOC_DEBUG=1 cmake --build . --target html_docs 2>&1 |tee log.txt && atom log.txt

The log.txt has this kind of entries (and a lot more, tried to pick some representative errors here):

qchar.h:45:10: error: 'functional' file not found
qchar.h:612:56: error: use of undeclared identifier 'std'
...
qbytearray.h:116:43: error: unknown type name 'qsizetype'
...
qscxmltabledata_p.h:59:10: error: 'functional' file not found
qscxmltabledata_p.h:73:18: error: no template named 'function' in namespace 'std'
...

These issues do not appear on Linux or on Windows - seems specific to Mac.