Hello! Looks like compiling this requires adding #include <unistd.h> in icnsutils/png2icns.c. Otherwise compilation fails with:
png2icns.c:328:25: error: implicit declaration of function ‘unlink’ [-Wimplicit-function-declaration]
328 | unlink(argv[1]);
Oh, I see that this was already fixed in de0ae9ab - It would be nice to get a new release cut for packagers!