c-block-comment-prefix

c-block-comment-prefix is a variable defined in `cc-vars.el'.
Its value is

set-from-style

Documentation:
*Specifies the line prefix of continued C-style block comments.
You should set this variable to the literal string that gets inserted
at the front of continued block style comment lines. This should
either be the empty string, or some characters without preceding
spaces. To adjust the alignment under the comment starter, put an
appropriate value on the `c' syntactic symbol (see the
`c-offsets-alist' variable).

It's only used when a one-line block comment is broken into two or
more lines for the first time; otherwise the appropriate prefix is
adapted from the comment. This variable is not used for C++ line
style comments.

This is a style variable. Apart from the valid values described
above, it can be set to the symbol `set-from-style'. In that case,
it takes its value from the style system (see `c-default-style' and
`c-style-alist') when a CC Mode buffer is initialized. Otherwise,
the value set here overrides the style system (there is a variable
`c-old-style-variable-behavior' that changes this, though).

You can customize this variable.