Clean up handling of missing functions.

With autotools, CMake, and nmake, if we have a function, #define
HAVE_{function_name_in_all_caps}, otherwise don't #define it.

If we provide our own version of a function in libwsutil, make sure we
have a header that declares it, and *ONLY* include that header if
HAVE_{function_name_in_all_caps} is *NOT* defined, so that we don't have
the system declaration and our declaration colliding.

Check for inet_aton, strncasecmp, and strptime with CMake, just as we do
with autotools.

Simplify the addition of {function_name_in_all_caps}_LO to libwsutil in
autotools.

Change-Id: Id5be5c73f79f81919a3a865324e400eca7b88889
Reviewed-on: https://code.wireshark.org/review/2903
Reviewed-by: Guy Harris <guy@alum.mit.edu>