Put back the check for gzseek() when building with the GTK+ build flags

Put back the check for gzseek() when building with the GTK+ build flags
(well, the check used to be a check for gzgets(), but we now have our
own version of that), so that, on a system with multiple versions of
zlib where the GTK+ -L flags get you an old version without gzgets(),
the configure fails, rather than the build failing.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@4566 f5534014-38df-0310-8fa8-9805f1628bb7