in src/main/java/org/apache/maven/shared/invoker/MavenCommandLineBuilder.java [245:257]
    protected void setProperties(InvocationRequest request, Commandline cli) {
        Properties properties = request.getProperties();
        if (properties != null) {
            for (Entry<Object, Object> entry : properties.entrySet()) {
                String key = (String) entry.getKey();
                String value = (String) entry.getValue();
                cli.createArg().setValue("-D");
                cli.createArg().setValue(key + '=' + value);
            }
        }
    }