c-identifier-syntax-modifications is a variable defined in `cc-engine.el'.
Its value is

  • Automatically becomes buffer-local when set.

A list that describes the modifications that should be done to the
mode syntax table to get a syntax table that matches all identifiers
and keywords as words.

The list is just like the one used in `font-lock-defaults': Each
element is a cons where the car is the character to modify and the cdr
the new syntax, as accepted by `modify-syntax-entry'.