In most cases instead pass `for_machine`, the name of the relevant
machines (what compilers target, what targets run on, etc). This allows
us to use the cross code path in the native case, deduplicating the
code.
As one can see, environment got bigger as more information is kept
structured there, while ninjabackend got a smaller. Overall a few amount
of lines were added, but the hope is what's added is a lot simpler than
what's removed.
Now, all source files are ignored that are not part of the
language of the target. This is also what CMake does.
Additionally it is now supported to build source files that
are generated inside the build directory.
Daniel Mensinger