libselinux: harden availability check against user CFLAGS

If CFLAGS set by the user contains the warnings override
`-Wno-error=implicit-function-declaration` the availability check does
not work properly.  Explicitly enable and treat this warnings as failure
by appending the appropriate flag.

Also include CPPFLAGS in the check.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: James Carter <jwcart2@gmail.com>

libselinux/src/Makefile

@@ -104,13 +104,13 @@ override CFLAGS += -I../include -D_GNU_SOURCE $(DISABLE_FLAGS) $(PCRE_CFLAGS)
 
 # check for strlcpy(3) availability
 H := \#
-ifeq (yes,$(shell printf '${H}include <string.h>\nint main(void){char d[2];const char *s="a";return (size_t)strlcpy(d,s,sizeof(d))>=sizeof(d);}' | $(CC) $(CFLAGS) $(LDFLAGS) -x c -o /dev/null - >/dev/null 2>&1 && echo yes))
+ifeq (yes,$(shell printf '${H}include <string.h>\nint main(void){char d[2];const char *s="a";return (size_t)strlcpy(d,s,sizeof(d))>=sizeof(d);}' | $(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -Werror=implicit-function-declaration -x c -o /dev/null - >/dev/null 2>&1 && echo yes))
 override CFLAGS += -DHAVE_STRLCPY
 endif
 
 # check for reallocarray(3) availability
 H := \#
-ifeq (yes,$(shell printf '${H}include <stdlib.h>\nint main(void){return reallocarray(NULL,0,0)==NULL;}' | $(CC) $(CFLAGS) $(LDFLAGS) -x c -o /dev/null - >/dev/null 2>&1 && echo yes))
+ifeq (yes,$(shell printf '${H}include <stdlib.h>\nint main(void){return reallocarray(NULL,0,0)==NULL;}' | $(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -Werror=implicit-function-declaration -x c -o /dev/null - >/dev/null 2>&1 && echo yes))
 override CFLAGS += -DHAVE_REALLOCARRAY
 endif