Involve libpng16 to get it compiled on a modern distro

[rmast]

I made a new PR ArtifexSoftware#4

I spent some time getting this repo built on Mint 20.3. It required to explicitly depend on libpng16 instead of libpng to get it compiled.

When I now compile it with ./autogen.sh CFLAGS=-DDUMP_SYMDICT=1 I finally get the symbols found in the document als little .png-files, as the dump function is not yet implemented.

https://stackoverflow.com/questions/72361820/how-to-read-out-the-properties-of-the-symbol-dictionary-used-by-the-jbig2-algori