To build png2ico on a Unix-like platform simply issue the command

    make
    
You need to have libpng installed. Furthermore, the Makefile uses the
GNU C++ compiler g++ by default. If you don't have g++, change the Makefile to
use your compiler instead.

There should be no problems building the program on a GNU/Linux system. If you
can provide information about building on other platforms, let me know.

The Makefile does not have an install target. To install the program, just
copy the binary png2ico to wherever you want it (e.g. /usr/local/bin/) and
the manpage doc/png2ico.1 to an appropriate directory 
(e.g. /usr/local/man/man1/).