Warning if check kwarg of run_command is missing

The check kwarg of run_command currently defaults to false. Because we want to change that, running

run_command('cmd')

now results in:

WARNING: You should add the boolean check kwarg to the run_command call.
         It currently defaults to false,
         but it will default to true in future releases of meson.
         See also: https://github.com/mesonbuild/meson/issues/9300