Define INET6 in config.h rather than in the Makefile.

That's what we do with nmake and CMake; this makes it uniform, so it's
easier to compare compiler commands.

Change-Id: Ice2c4bd796e921fa3089e8a45a32ad14d9b0e0a4
Reviewed-on: https://code.wireshark.org/review/3387
Reviewed-by: Guy Harris <guy@alum.mit.edu>