deps: fix zlib -Wimplicit-function-declaration

Build the bundled zlib with -DZ_HAVE_UNISTD_H to make the definition of close(), read() and other unistd.h functions available to gzread.c and gzwrite.c. It's kind of silly that we have to jump through hoops here because we never call any of the functions that do I/O directly, but at least it squelches the -Wimplicit-function-declaration warnings.

R=@piscisaureus or @shigeki