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).

This should be ok for Boost > 1.33 and was tested with Boost >= 1.53.

silences a compiler warning in gcc 8.3 and is a good idea anyway

These are only updates to documentation.

Tested with Ubuntu 18.04 (in a vagrant box) following instructions in doc and
adding cmake flags "-DENABLE_STATIC=ON -DENABLE_INFO=OFF". Tested resulting
binaries "chemdict_tool" and "lddt" in Ubuntu-box and host-system (CentOS 7).