Huhu, I found the "obvious" other solution first; the /**/ thing was an afterthought. I'm pretty sure that whitespaceless C is turing-complete, even without preprocessor tricks like comments.
"Whitespace" is a sequence of 1 or more white characters. White characters recognized by the judge include: ' ' (space), '\t', (tab), '\n' (newline), '\r' (carriage return), '\f' (form feed) and '\v' (vertical tab). No other ASCII char is white, but there may be other white characters such as char 160 (non-breaking space); I haven't tested those.
A general comment: '\0' (NUL) isn't whitespace. It can't be used to separate tokens either. (I don't care if this works in gcc.)