flymake-warning-predicate is a variable defined in `flymake.el'.
Its value is
"^[wW]arning"
This variable may be risky if used as a file-local variable.
Documentation:
Predicate matching against error text to detect a warning.
Takes a single argument, the error's text and should return non-nil
if it's a warning.
Instead of a function, it can also be a regular expression.