This is useful when build_machine appears to be compatible with
host_machine, but actually isn't. For example when:
- build_machine is macOS and host_machine is the iOS Simulator
- the build_machine's libc is glibc but the host_machine libc is uClibc
- code relies on kernel features not available on the build_machine
This allows us to more easily have the documentation in sync with
the source code as people will have to document new features etc
right at the time where they implement it.
Jussi Pakkanen