c-constant-symbol is a compiled Lisp function in `cc-vars.el'.

(c-constant-symbol SYM LEN)

Create an uneditable symbol for customization buffers.
SYM is the name of the symbol, LEN the length of the field (in
characters) the symbol will be displayed in. LEN must be big

This returns a (const ....) structure, suitable for embedding
within a customization type.