configure: Split adding of CFLAGS and CPPFLAGS for hardened toolchain

configure

@@ -2553,8 +2553,9 @@ case "$toolchain" in
         add_ldflags -fprofile-arcs -ftest-coverage
     ;;
     hardened)
-        add_cflags  -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fno-strict-overflow -fstack-protector-all
-        add_ldflags -Wl,-z,relro -Wl,-z,now
+        add_cppflags -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2
+        add_cflags   -fno-strict-overflow -fstack-protector-all
+        add_ldflags  -Wl,-z,relro -Wl,-z,now
     ;;
     ?*)
         die "Unknown toolchain $toolchain"