zlib: fix include of zlib.h

Including <zlib.h> may lead to false positives when the user specifies a bad
path in `./configure --shared-zlib --shared-zlib-includes=/path/to/zlib`. If
a zlib.h exists somewhere on the system include path (common on UNIX systems),
the compiler will include that instead, possibly leading to header mismatch
errors that are hard to debug.