This will copy the file to the build directory without trying to read
it or substitute values into it.
Also do this optimization if the configuration_data() object passed to
the `configuration:` kwarg is empty, and print a warning about it.
See also: https://github.com/mesonbuild/meson/issues/1542
This already reports the location (in a slightly different format), but
using mlog.warning() will make it easier if we want to change the location
format in future.