'native:' keyword was only added in 0.54. For projects declaring
meson_version >= 0.54, warn, because those projects can and should set
the keyword. For older projects declaring support for older versions,
don't warn and use the default implicitly.

Fixes https://github.com/mesonbuild/meson/issues/6849.

The intended use it to tell people the new thing to do.

Initially produced using:

  for d in "test cases/failing/"* ; do rm -r _build ; ./meson.py setup "$d" _build | grep ERROR >"$d"/expected_stdout.txt; done

then converted to json with jq using:

  jq --raw-input --slurp 'split("\n") | {stdout: map({line: select(. != "")})}' expected_stdout.txt >test.json

or merged with existing json using:

  jq --slurp '.[0] + .[1]' test.json expected.json >test.json.new

v2:
Add some comments to explain the match when it isn't totally obvious

v3:
Add or adjust existing re: in expected output to handle '/' or '\' path
separators appearing in message, not location.

v4:
Put expected stdout in test.json, rather than a separate expected_stdout.txt file
Park comments in an unused 'comments' key, as JSON doesn't have a syntax for comments

Meson is not warning if you join paths with / but you are requesting a
version older than 0.49.0; fix this before adding more features to the
division operator.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>