help: print zlib-ng version number

help: print zlib-ng version number

When building against zlib-ng, the header file `zlib.h` is not included,
but `zlib-ng.h` is included instead. It's `zlib.h` that defines
`ZLIB_VERSION` and that macro is used to print out zlib version in
`git-version(1)` with `--build-options`. But when it's not defined, no
version is printed.

`zlib-ng.h` defines another macro: `ZLIBNG_VERSION`. Use that macro to
print the zlib-ng version in `git version --build-options` when it's
set. Otherwise fallback to `ZLIB_VERSION`.

Signed-off-by: Toon Claes <toon@iotcl.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>