This could be a return of parts of commit f384c34f67a3544dcabf23389a6089f897340c8b
Without these, the rc1 build fails on FreeBSD with "pybind11/eigen.h not
found" - the layout of the pybind11 directories on the BSDs may differ
somewhat from your typical Linux distribution. But then, on Linux, this
additional include path will just point to an already-known location.
f382090ab5