Dinamically linked pngtest.exe succeeds now. All it took was to
recompile and relink zlib, libpng and pngtest with the /br option,
asking Open Watcom to use the DLL version of its runtime, so now they
share the same copy of the I/O code. The C runtime consists of two
redistributable files, clbr20.dll (C library) and mt7r20.dll (Math
library), which need to be in path.
Thank you very much, again, for your help.