Only ABI-check strncasecmp.h if strncasecmp() is part of the API/ABI.

Some routines Wireshark uses are present in some, but not all,
platforms; for routines that would be used on all platforms, libwsutil
provides its own implementations on platforms that lack them.

On platforms that provide a routine, that routine will not be part of
the API and ABI, and, if we do an API or ABI check using the header
libwsutil provides to declare the function on platforms that lack it, we
may have a collision between the declaration in our header and the
declaration in a system header.

There's no guarantee that we can make them match, as the declaration
might differ from platform to platform and from platform version to
platform version, so we simply leave the header file out of the check if
we have the function on the platform on which we're checking the API or
ABI.

Change-Id: I8a23e63d9e17e5c1f5a83304dbe14d1e7df22e7e
Reviewed-on: https://code.wireshark.org/review/3115
Reviewed-by: Guy Harris <guy@alum.mit.edu>