What steps will reproduce the problem?
1. Run with cppcheck 1.33 (included in current ubuntu release)
2. Get lots of errors about content not being allowed in prolog
3.

What is the expected output? What do you see instead?

You're already checking the cppcheck version to determine if there's an
update (that works fine), perhaps you should add an extra warning if you
encounter version < 1.37 and tell people that it is not going to work anyway :)