What’s a portable way to implement no-op statement in C++?

The simplest no-op is just having no code at all:

#define noop

Then user code will have:

if (condition) noop; else do_something();

The alternative that you mention is also a no-op: (void)0;, but if you are going to use that inside a macro, you should leave the ; aside for the caller to add:

#define noop (void)0
if (condition) noop; else do_something();

(If ; was part of the macro, then there would be an extra ; there)