libxml2 knows too much about zlib internals. Beginning from zlib 188.8.131.52
the zlib authors has provided a 'gzdirect' function to detect whether the
current gzFile descriptor is still providing a stream from uncompression,
so use it instead of rolling own.
The upcoming zlib 1.2.4 update will break the current libxml2 usage.
Patch was the same as Mark Adler provided to libxml2 maintainers at:
Approved by: mezz (freebsd-gnome@)