The same as in C.
int i;
for(i=0; i<100; i++)
{
}
In C (unlike C++ and Java), you have to declare your variables at the beginning of a block; declaring them inline like that is not supported.
The recent ANSI C99 spec does change the C language spec to allow declaring of inline variables (at least in for loops such as yours), but C99 compliance has not been integrated into Apple's cc yet. I'm sure it will get there eventually, but it's not there now.