The includes magic changes I made for the tests were done quite carefully,
and I did test them (using runcheck but not installcheck), so I'm surprised
you had issues I didn't see. It's all rather subtle, so I think it's wise
to discuss any such changes rather than just sweep them in. (I suppose I
should have done so.) And, frankly, for anything that even might either be
subtle or ever be important, commits with a log explanation of "Tweak it"
are just inadequate.
Those paths need to be exactly just right to have installcheck actually
test the installed headers so we can know they got installed properly,
which is its purpose. For the runcheck case, they need to be differently
exactly just right to have it find the right headers in the build and
source directories and no others. For both cases, it's important that they
be found in an -isystem path so that -pedantic doesn't emit meaningless
stupid warnings that there is no other way to suppress (until GCC is fixed).