As weird as it sounds but the proper var. to set is PNG_PNG_INCLUDE_DIR instead
of PNG_INCLUDE_DIR. The latter is overwritten by FindPNG and actually obsolete
(see https://github.com/Kitware/CMake/blob/master/Modules/FindPNG.cmake).

hasn't been used/tested for years

Currently active Fedora is 29

The reason is that we want to profit from latest security updates and
therefore not use a copy of the SQLite3 source code that becomes outdated.

ToDo: test on different Linux distributions / bc2 and adapt compile
instructions accordingly, also container recipes must be adapted.