pngpp: darwin support

1) Building with clang is addressed by navigating a minor #if in some
of the code.

2) I noticed that even when things were building correctly, passing
`${out}` as a variable assignment to `make` was actually not working:
there were compiler warnings about missing include directories whose
bogus paths involved the literal string `out`. I ended up fixing this
by performing the assignment to the make variable `PREFIX` in the
`Makefile` itself.