Tools.Standalone.cflags prints master()->cflags, which typically equals "-DDYNAMIC_MODULE -I" + dirname(master()->include_prefix). Tools.Standalone.module uses master()->include_prefix directly, however.
Explanation? When is Tools.Standalone.cflags meant to be used? "New style" modules? Are sources relying on master()->cflags to be compiled meant to #include <pike/whatever> instead of #include <whatever>?