target_header_dir vs host-x-host

[DJ Delorie]

configure has various ways of specifying the target headers for a
cross-compiler.  However, none of these work when you're
cross-building a native (build!=host==target).  Unfortunately,
configure looks in $target_header_dir for target headers to determine
various bits of functionality.

What is the right way to tell the cross compiler which is
cross-building the native, where the cross-built native headers are?