Make illumos build warning-free. [skip appveyor]

Make illumos build warning-free. [skip appveyor]

As it turns out, the root cause of Clang warnings about tcpdump.c on
illumos has to do with illumos headers rather than tcpdump.  Until the
bug fix is in place and has propagated well into illumos distributions,
use a workaround: add a new pair of diagnostic control macros for Clang
only and on illumos wrap the "offending" lines with it.

(cherry picked from commit 68761b465f6db19d5f5da9481322c74629c6e7f9)