cppcheck: Use "exhaustive" level checking when available

Recent enough cppcheck versions (at least as of cppcheck 2.12) give this
error processing conf.c:

conf.c:1179:1: information: ValueFlow analysis is limited in conf. Use --check-level=exhaustive if full analysis is wanted. [checkLevelNormal]

Adding --check-level=exhaustive doesn't seem to significantly increase the
cppcheck run time for us, so enable it when possible, suppressing that
warning.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Makefile

@@ -278,6 +278,11 @@ clang-tidy: $(SRCS) $(HEADERS)
 	-config='{CheckOptions: [{key: bugprone-suspicious-string-compare.WarnOnImplicitComparison, value: "false"}]}' \
 	--warnings-as-errors=* $(SRCS) -- $(filter-out -pie,$(FLAGS) $(CFLAGS) $(CPPFLAGS)) -DCLANG_TIDY_58992
 
+CPPCHECK_EXHAUSTIVE :=
+ifeq ($(shell cppcheck --check-level=exhaustive /dev/null > /dev/null 2>&1; echo $$?),0)
+	CPPCHECK_EXHAUSTIVE += --check-level=exhaustive
+endif
+
 SYSTEM_INCLUDES := /usr/include $(wildcard /usr/include/$(TARGET))
 ifeq ($(shell $(CC) -v 2>&1 | grep -c "gcc version"),1)
 VER := $(shell $(CC) -dumpversion)
@@ -286,6 +291,7 @@ endif
 cppcheck: $(SRCS) $(HEADERS)
 	cppcheck --std=c11 --error-exitcode=1 --enable=all --force	\
 	--inconclusive --library=posix --quiet				\
+	$(CPPCHECK_EXHAUSTIVE)						\
 	$(SYSTEM_INCLUDES:%=-I%)					\
 	$(SYSTEM_INCLUDES:%=--config-exclude=%)				\
 	$(SYSTEM_INCLUDES:%=--suppress=*:%/*)				\