aicasm build error

For a while I've been getting a build error on aicasm during the early
steps of the build.  Today I finally dug into it and discovered it's
because the build is using a mix of header files, some from /usr/include
on the build host, and some from obj/... for the target being built.  It
happens because dev/aic7xxx/aicasm/Makefile contains CFLAGS+= -I. and
the current directory when aicasm builds is obj/.../<kernconf>.  In my
case it was picking up machine/_types.h from objdir.

Apparently this is usually harmless, but if the build host system and
the target versions are far enough apart, or with a cross-build where
the host and target are different architectures, you can get problems.  

After a little digging, I just now discovered Marcel fixed this long ago
in r70000, but then it got undone in r236578 a couple months ago.  Is
there any reason not to restore Marcel's fix, such as with the attached
patch?

-- Ian