* [docu]: update README to oS-13.2
* [conf]: HAVE_DIRENT_H needs to be visible in sources

prepare libpng variable
* [oPNG]: select fitting libpng header

on oS-13.2 exist libpng12 and libpng16 headers in parallel with
png.h in /usr/include. That causes libpng to compile against different
header version than to link with a library version.