The include directory usage requirement for the build tree was added by
commit 993f0ea4 (Add an option to split object and interface libs,
2017-06-19) and preserved by commit b61faf89 (Fix install rules and
usage requirements for interface+object libs, 2017-06-28).  However, its
value should be that of `KWSYS_HEADER_ROOT` rather than just its
default.