[9front] zlib update

[Lucas Francesco <lucas.francesco93@gmail.com>]

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Today people asked me to resubmit the zlib patches that I had sent
months ago due to netsurf/nsfb. I actually had things working and
clean on my system since back then, but I was confused between a
decision of sending the build "half-broken" but with the final result
working, or triggering a flag up that I didn't know if it should or
not, after looking up, it seems the cleaner route is just to do the
latter.

the newer zlib uses lseek/open/write, functions specified in
<unistd.h>, but it actually doesn't include the header when building
some of the .$O, what happened it was just that our linker would spot
the functions even after the compiler warning due to the lax
prototypeless functions in ANSI C std and build it fine. (the -B flag
on 2c would do it)

the option I'm going here is just to make it include <unistd.h>, that
requires anything that uses ape/zlib to define _POSIX_SOURCE, all
tests I've done points out it is working fine, page works, the zlib
tests passes and netsurf png decoding work as always with it.

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