Makefile: check for cppcheck's --check-level option in cppcheck target

Don't run cppcheck to find out if the --check-level=exhaustive option
is available, unless we're actually going to run cppcheck later.

To avoid this, move this check under the cppcheck target, and
implement it in shell script instead of using Makefile directives,
because we can't easily implement conditionals in recipes.

Reported-by: Rahil Bhimjiani <me@rahil.website>
Link: https://bugs.gentoo.org/920795
Fixes: 8640d62af7 ("cppcheck: Use "exhaustive" level checking when available")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Makefile

@@ -287,20 +287,20 @@ 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 $$?),0)
-	CPPCHECK_EXHAUSTIVE += --check-level=exhaustive
-endif
-
 SYSTEM_INCLUDES := /usr/include $(wildcard /usr/include/$(TARGET))
 ifeq ($(shell $(CC) -v 2>&1 | grep -c "gcc version"),1)
 VER := $(shell $(CC) -dumpversion)
 SYSTEM_INCLUDES += /usr/lib/gcc/$(TARGET)/$(VER)/include
 endif
 cppcheck: $(SRCS) $(HEADERS)
+	if cppcheck --check-level=exhaustive /dev/null > /dev/null 2>&1; then \
+		CPPCHECK_EXHAUSTIVE="--check-level=exhaustive";		\
+	else								\
+		CPPCHECK_EXHAUSTIVE=;					\
+	fi;								\
 	cppcheck --std=c11 --error-exitcode=1 --enable=all --force	\
 	--inconclusive --library=posix --quiet				\
-	$(CPPCHECK_EXHAUSTIVE)						\
+	$${CPPCHECK_EXHAUSTIVE}						\
 	$(SYSTEM_INCLUDES:%=-I%)					\
 	$(SYSTEM_INCLUDES:%=--config-exclude=%)				\
 	$(SYSTEM_INCLUDES:%=--suppress=*:%/*)				\