Include some files based on whether the functions they define are present.

Rather than including wsgetopt.c, inet_aton.c, and strptime.c iff we're
building for Windows:

	include wsgetopt.c iff we don't have getopt();

	include inet_aton.c iff we don't have inet_aton();

	include strptime.c iff we don't have strptime().

Change-Id: Ibd68306ac372a4ae102c3220a94cdf6ecb04e58c
Reviewed-on: https://code.wireshark.org/review/6044
Reviewed-by: Guy Harris <guy@alum.mit.edu>