cmake: use pkg-config for resolving dependencies

Changes:
 * Fix glib2 search path.
 * Add pkg-config support to CAP, GEOIP, GNUTLS, LUA, PortAudio (API
   19), zlib, kerberos.
 * Add pkg-config support to libnl3, libnl2 and libnl1 (but tested only
   with libnl3).

This makes it easier to do 32-bit builds on 64-bit hosts by just setting
`PKG_CONFIG_LIBDIR`. Due to how HINTS work, it is still fragile though:
missing 32-bit libraries will cause a fallback to 64-bit libraries.

A future patch could check for `<PREFIX>_FOUND` and remove the manual
`find_path` and `find_library` hackery since the paths are already
known.

Change-Id: Ieb4fb74695c96afb1a4c70168e84abb1fa4612c6
Reviewed-on: https://code.wireshark.org/review/4292
Reviewed-by: Anders Broman <a.broman58@gmail.com>
(cherry picked from commit 9506909223f6155085a7a4d5adc039ebf60030f2)
Reviewed-on: https://code.wireshark.org/review/4399
Reviewed-by: Michael Mann <mmann78@netscape.net>