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>