cwarn-configuration is a variable defined in `cwarn.el'.
Its value is
((c-mode (not reference)) (c++-mode t))

List of items each describing which features are enable for a mode.
Each item is on the form (mode featurelist), where featurelist can be
on one of three forms:

* A list of enabled features.
* A list starting with the atom `not' followed by the features
which are not enabled.
* The atom t, that represent that all features are enabled.

See variable `cwarn-font-lock-feature-keywords-alist' for available

You can customize this variable.