Here's what I ended up writing:
const char* ordinal_suffix(int n)
{
static const char suffixes [][3] = {"th", "st", "nd", "rd"};
auto ord = n % 100;
if (ord / 10 == 1) { ord = 0; }
ord = ord % 10;
if (ord > 3) { ord = 0; }
return suffixes[ord];
}
The code golf solutions are cute, but - they really do optimize for terseness, not anything else. This is faster (although it could be made even faster by putting the suffixes in a .cpp out of the function body and making the code inlinable), much clearer, and still more terse than most other answers here.