No, djgpp is not public domain. It is copyrighted. Some parts of it are based on public domain sources, but that does not mean that djgpp itself is public domain. The specific sources are usually in libc.a or libm.a, and are individually labelled with their copyright. You may do what you like with sources specifically labelled ``public domain'' in their copyright, but you must comply with the GPL or djgpp's copyright for everything else.