- was useful to overcome submodule error for 'pybind11' in github.com/tdegeus/pybind11_examples
- the command mentioned in Edit 2, 'git submodule sync' is crucial
Have tried following the dependency for: libmotif-dev → libxft-dev → libfontconfig1-dev → libfreetype6-dev → libpng-dev
where finally resolved by 'apt install libpng-dev' which removed an older installed packge, libpng16-dev and replaced with newer libpng12-dev