Disambiguate the grammar for backtick. The old syntax suggested that a trailing comma was OK inside backticks, but in fact (due to ideosyncrasies of pgen) it was not. Fix the grammar to avoid the ambiguity. Fred: you may want to update the refman.

#include "Python.h"

#ifndef PLATFORM
#define PLATFORM "unknown"
#endif

const char *
Py_GetPlatform(void)
{
	return PLATFORM;
}