Always honour any windres setting in cross-file (we can't be compiling with
msvc, but this should apply when cross-compiling using gcc or clang)
Always honour WINDRES environment variable
Otherwise look for the resource compiler which is part of the same toolset
as the C or C++ compiler.
Add some commentary on why the conventions for compiled resource file
extensions differ between RC and windres
Also don't try to report non-existent path when we couldn't find the
resource compiler.
Expose depend_files: from the custom_target this creates.
This is the change suggested in #2815, with tests and documentation added.
Fixes#2789 (duplicate #2830)
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.
Jon Turney