When building FreeBSD the makefiles invoke find with various flags such as
`-s` that aren't supported in the native /usr/bin/find. To fix this I
build the FreeBSD version of find and use that when crossbuilding.
Inserting lots if #ifdefs in the code is rather ugly but I don't see a
better solution.
Reviewed By: brooks (mentor)
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D13306
4388a2f09a