Suppress "casting away const" warnings in missing/getopt_long.c.

Suppress "casting away const" warnings in missing/getopt_long.c.

It's explicitly cheating in some places; just cast away the warnings.

Add a Clang version testing macro to compiler-tests.h.

Move the diagnostic control #defines from netdissect-stdinc.h to
diag-control.h, libpcap-style, so that they can be used by code that
doesn't use (or need) netdissect-stdinc.h.  This also means that we can
limit the inclusion of diag-control.h, and the definition of those

Do them more libpcap-style, with separate DIAG_OFF/DIAG_ON pairs for
particular issues, rather than having DIAG_OFF() and DIAG_ON() macros
that take -W option names as arguments; that way, if we need to define
them for compilers that don't have the GCC/Clang syntax for those
pragmas, e.g. MSVC, we can do so.

(cherry picked from commit 39f09d68ce7ebe9e229c9bf5209bfc30a8f51064)