c-indent-comments-syntactically-p is a variable defined in `cc-vars.el'.
Its value is

*Specifies how M-x indent-for-comment should handle comment-only lines.
When this variable is non-nil, comment-only lines are indented
according to syntactic analysis via `c-offsets-alist'. Otherwise, the
comment is indented as if it was preceded by code. Note that this
variable does not affect how the normal line indentation treats
comment-only lines.

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.