This demonstrates the use of PNG_USER_CONFIG, pngusr.h and pngusr.dfa The makefile builds a minimal read-only decoder with embedded libpng and zlib. Specify the location of the zlib source (1.2.1 or later) as ZLIBSRC on the make command line. If you prefer to use the shared libraries, go to contrib/pngminus and build the png2pnm application there.