Include string.h for str{cat,cpy,len,ncpy}
Don't overlink to libpng
Size out_fn buffer correctly when encoding
Respect LDFLAGS
Don't use floating point arithmetic
- Fixed Makefile target 'test' to require target 's2png'.
- Show compiler warnings by default; specify C standard.
- Only recursively delete the dSYM directory. (Gave rm(1) a condom!)