git-compat-util: add NOT_CONSTANT macro and use it in atfork_prepare()

Our hope is that the number of code paths that falsely trigger
warnings with the -Wunreachable-code compilation option are small,
and they can be worked around case-by-case basis, like we just did
in the previous commit. If we need such a workaround a bit more
often, however, we may benefit from a more generic and descriptive
facility that helps document the cases we need such workarounds.

Side note: if we need the workaround all over the place, it
simply means -Wunreachable-code is not a good tool for us to
save engineering effort to catch mistakes. We are still
exploring if it helps us, so let's assume that it is not the
case.

Introduce NOT_CONSTANT() macro, with which, the developer can tell
the compiler:

Do not optimize this expression out, because, despite whatever
you are told by the system headers, this expression should *not*
be treated as a constant.

and use it as a replacement for the workaround we used that was
somewhat specific to the sigfillset case. If the compiler already
knows that the call to sigfillset() cannot fail on a particular
platform it is compiling for and declares that the if() condition
would not hold, it is plausible that the next version of the
compiler may learn that sigfillset() that never fails would not
touch errno and decide that in this sequence:

errno = 0;
sigfillset(&all)
if (errno)
die_errno("sigfillset");

the if() statement will never trigger. Marking that the value
returned by sigfillset() cannot be a constant would document our
intention better and would not break with such a new version of
compiler that is even more "clever". With the marco, the above
sequence can be rewritten:

if (NOT_CONSTANT(sigfillset(&all)))
die_errno("sigfillset");

which looks almost like other innocuous annotations we have,
e.g. UNUSED.

Signed-off-by: Junio C Hamano <gitster@pobox.com>