[PATCH 02/13] cppcheck: Split out essential defines into a BASE_CPPFLAGS variable

[David Gibson <david@gibson.dropbear.id.au>]

Our cppcheck target need certain flags from the compiler so that they it
can analyse the code correctly.  Currently we extract these rather
awkwardly from FLAGS / CFLAGS / CPPFLAGS.  But this means we inhibit one
of cppcheck's features: by default it will attempt to analyse paths for all
combinations of compile time options, not just a single one.

Analysing *all* paths doesn't work for us because many of the -D options we
use are essential to compile at all, so unless we supply those to cppcheck,
overriding the default behaviour we get many spurious errors.  At the
moment, however, we give cppcheck *all* our -D options, including
conditional / configurable ones, not just the essential ones.

All cppcheck really needs here is those essential -D options.  Split those
into a separate variable, and use that directly rather than the clunky
$(filter) expression.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 Makefile | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index 17e70d22..0de98375 100644
--- a/Makefile
+++ b/Makefile
@@ -30,11 +30,15 @@ ifeq ($(shell $(CC) -O2 -dM -E - < /dev/null 2>&1 | grep ' _FORTIFY_SOURCE ' > /
 FORTIFY_FLAG := -D_FORTIFY_SOURCE=2
 endif
 
+# Require preprocessor flags we can't build without
+BASE_CPPFLAGS := -D_XOPEN_SOURCE=700 -D_GNU_SOURCE \
+	-DPAGE_SIZE=$(shell getconf PAGE_SIZE) \
+	-DVERSION=\"$(VERSION)\"
+
 FLAGS := -Wall -Wextra -Wno-format-zero-length -Wformat-security
-FLAGS += -pedantic -std=c11 -D_XOPEN_SOURCE=700 -D_GNU_SOURCE
+FLAGS += -pedantic -std=c11
 FLAGS +=  $(FORTIFY_FLAG) -O2 -pie -fPIE
-FLAGS += -DPAGE_SIZE=$(shell getconf PAGE_SIZE)
-FLAGS += -DVERSION=\"$(VERSION)\"
+FLAGS += $(BASE_CPPFLAGS)
 FLAGS += -DDUAL_STACK_SOCKETS=$(DUAL_STACK_SOCKETS)
 
 PASST_SRCS = arch.c arp.c bitmap.c checksum.c conf.c dhcp.c dhcpv6.c \
@@ -195,6 +199,4 @@ CPPCHECK_FLAGS = --std=c11 --error-exitcode=1 --enable=all --force	\
 	 -D CPPCHECK_6936
 
 cppcheck: $(PASST_SRCS) $(HEADERS)
-	$(CPPCHECK) $(CPPCHECK_FLAGS) 					\
-		$(filter -D%,$(FLAGS) $(CFLAGS) $(CPPFLAGS)) $^		\
-		$^
+	$(CPPCHECK) $(CPPCHECK_FLAGS) $(BASE_CPPFLAGS) $^
-- 
2.53.0