fixinc w/ SYSTEM_INCLUDE_DIR set?

[Robert Lipe]

I'm working on a target that will need to set SYSTEM_INCLUDE_DIR.  There
is a /usr/include, but they are toxic to this target and must never be
touched.   Unfortunately, the target of SYSTEM_INCLUDE_DIR is also toxic
enough that it needs to have fixincludes run on it.

I think what I need to be able to do is to set SYSTEM_HEADER_DIR in
Makefile.in from within configure.  I don't see any mechanism in there
to do this.  Is the right thing to do just to add the plumbing in
configure.in to AC_SUBST() it into the generated Makefile?

Since I can't believe I'm the first target to need this, I have to
believe that I'm missing the way to do this. :-)

Any ideas?

RJL