Adds makefiles include directory to recursive make invocations

[dbosk]

Whenever we call MAKE in subdir.mk we will add -I ${INCLUDE_MAKEFILES}. This allows us to have makefiles in the subdirectories which have not set INCLUDE_MAKEFILES (e.g. old MIUN makefiles).

[dbosk]

Maybe this would be nice to have in general? E.g.

../%:
    ${MAKE} -I ${INCLUDE_MAKEFILES} -C $(dir $@) $(notdir $@)

might work. Or more generally

%:
    [ -z "$(dir $@)" ] && ${MAKE} -I ${INCLUDE_MAKEFILES} -C $(dir $@) $(notdir $@)

However, the latter must be placed at the very end of all includes, which is difficult to ensure in an include.

[dbosk]

A better solution (documented here):

MAKEFLAGS+= -I ${INCLUDE_MAKEFILES}

The question is in which include file to put this.