fix compile error on AIX/gcc-2.9 because of unknown std::stringstream
FindGTK2: search for Fontconfig and X11 before using them
Utilities/Sphinx: Fix cmake domain document removal with python3

In the domain clear_doc method, avoid removing entries from a dictionary
while iterating over it.  Instead accumulate a set of entries to remove
at the end.
Merge topic 'aix-streams'

817db64c fix compile error on AIX/gcc-2.9 because of unknown std::stringstream
Merge topic 'FindGTK2-X11'

55f5ea7b FindGTK2: search for Fontconfig and X11 before using them
Merge topic 'sphinx-python3'

d55671ad Utilities/Sphinx: Fix cmake domain document removal with python3