CMake: Use WIRESHARK_LIB_DIR. Fix GThread check on Windows.

Have FindWSWinLibs check for WIRESHARK_LIB_DIR before WIRESHARK_BASE_DIR
to match the behavior of config.nmake and the Windows buildbots, which
set the former but not the latter.

It looks like the latest GLib DLL contains GThread. Adjust
FindGTHREAD2.cmake accordingly.

Change-Id: I0232b1819eca9ba2c39ae6834daddf65af979d7d
Reviewed-on: https://code.wireshark.org/review/4887
Reviewed-by: Gerald Combs <gerald@wireshark.org>