I think that the correct solution to this problem is something like
   what jtc described, and what the X11 build does, which is that at the
   beginning of the build, you do a recursive ``make includes'', which
   fully populates /usr/src/include (or maybe
   /usr/src/${MACHINE}/include).   All subsequent steps in the build
   reference /usr/src/${MACHINE}/include rather than /usr/include.   This
   assures you of a clean build (at least insofar as includes are
   concerned) if you type ``make build.''

i *strongly* disagree with the idea of *building* things into
/usr/src/include.  i'm unsure about the idea as a whole at the
moment, but if it was adopted, it would need to build to some
other place.

.mrg.