If there's no "zlib.h" header, we won't be using zlib, so don't bother

If there's no "zlib.h" header, we won't be using zlib, so don't bother
checking for "gzgets()" in zlib.

If there is a "zlib.h" header, and there is a "gzgets()" in zlib, check
whether we find "gzgets()" in zlib when we link with the GTK+ link
flags, and, if not, fail.  People often grab XFree86 source and build
and install it on their systems, and they appear sometimes to
misconfigure XFree86 so that, even on systems with zlib, it assumes
there is no zlib, so the XFree86 build process builds and installs its
own "mini-zlib" in the X11 library directory.  The "mini-zlib" lacks
"gzgets()", and that's the zlib with which Ethereal gets linked, so the
build of Ethereal fails.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@3849 f5534014-38df-0310-8fa8-9805f1628bb7