Add logic to config.nmake to find 64-bit cygwin, and a note warning
about forward slashes in the QT5_BASE_PATH if the recommended qmake
command is used; both of these issues will cause annoyingly non-
obvious errors.

Change-Id: I6f9a662969d1fc1887e75e8b0fde3520bbbea416
Reviewed-on: https://code.wireshark.org/review/2749
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>