Include wsgetopt.c based on whether getopt() is present.

Rather than including wsgetopt.c iff we're building for Windows, include
it iff we don't have getopt().

Change-Id: Icd4bf28721f1db5d61117769e87ab97516d2dbf2
Reviewed-on: https://code.wireshark.org/review/6046
Reviewed-by: Guy Harris <guy@alum.mit.edu>