Autodiscover flex and bison in config.nmake.

Change-Id: I4683d0eeb06e5616103f3e6e25ce7f1549a2bb05
Reviewed-on: https://code.wireshark.org/review/3510
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>