- 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!)
Replaced MODE_ENCODE and MODE_DECODE with an anonymous enum. ^_^
Improve install target

Don't install to /usr/local/bin/bin by default, and honor DESTDIR.
Fix types and sizes in is_png_file