Fix cppcheck 1.89 warnings

Fix cppcheck 1.89 warnings

cppcheck 1.89 enabled certain value flow analysis mechanisms [1] which
trigger null pointer dereference false positives in lib/dns/rpz.c:

    lib/dns/rpz.c:584:7: warning: Possible null pointer dereference: tgt_ip [nullPointer]
      if (KEY_IS_IPV4(tgt_prefix, tgt_ip)) {
          ^
    lib/dns/rpz.c:1425:44: note: Calling function 'adj_trigger_cnt', 4th argument '(void*)0' value is 0
      adj_trigger_cnt(rpzs, rpz_num, rpz_type, NULL, 0, true);
                                               ^
    lib/dns/rpz.c:584:7: note: Null pointer dereference
      if (KEY_IS_IPV4(tgt_prefix, tgt_ip)) {
          ^
    lib/dns/rpz.c:598:7: warning: Possible null pointer dereference: tgt_ip [nullPointer]
      if (KEY_IS_IPV4(tgt_prefix, tgt_ip)) {
          ^
    lib/dns/rpz.c:1425:44: note: Calling function 'adj_trigger_cnt', 4th argument '(void*)0' value is 0
      adj_trigger_cnt(rpzs, rpz_num, rpz_type, NULL, 0, true);
                                               ^
    lib/dns/rpz.c:598:7: note: Null pointer dereference
      if (KEY_IS_IPV4(tgt_prefix, tgt_ip)) {
          ^
    lib/dns/rpz.c:612:7: warning: Possible null pointer dereference: tgt_ip [nullPointer]
      if (KEY_IS_IPV4(tgt_prefix, tgt_ip)) {
          ^
    lib/dns/rpz.c:1425:44: note: Calling function 'adj_trigger_cnt', 4th argument '(void*)0' value is 0
      adj_trigger_cnt(rpzs, rpz_num, rpz_type, NULL, 0, true);
                                               ^
    lib/dns/rpz.c:612:7: note: Null pointer dereference
      if (KEY_IS_IPV4(tgt_prefix, tgt_ip)) {
          ^

It seems that cppcheck no longer treats at least some REQUIRE()
assertion failures as fatal, so add extra assertion macro definitions to
lib/isc/include/isc/util.h that are only used when the CPPCHECK
preprocessor macro is defined; these definitions make cppcheck 1.89
behave as expected.

There is an important requirement for these custom definitions to work:
cppcheck must properly treat abort() as a function which does not
return.  In order for that to happen, the __GNUC__ macro must be set to
a high enough number (because system include directories are used and
system headers compile attributes away if __GNUC__ is not high enough).
__GNUC__ is thus set to the major version number of the GCC compiler
used, which is what that latter does itself during compilation.

[1] https://github.com/danmar/cppcheck/commit/aaeec462e6d96bb70c2b1cf030979d09e2d7c959

(cherry picked from commit abfde3d543576311ce5d32089d774a360b7edc9f)