Check for getopt_long(), not getopt().

We support three types of platforms:

	1) UN*Xes that have both getopt() and getopt_long();

	2) UN*Xes that have getopt() but not getopt_long();

	3) Windows, which has neither.

Checking for getopt_long() lets us distinguish between 1) and 2) and
build getopt_long() for them.

Change-Id: Iaf0f142f9bebaa2eed2128d544ec9786711def45
Reviewed-on: https://code.wireshark.org/review/6045
Reviewed-by: Guy Harris <guy@alum.mit.edu>