How do you include a header file that may or may not exist?

Let's assume I define BAR in foo.h. But foo.h might not exist. How do I include it, without the compiler complaining at me?

#include "foo.h"

#ifndef BAR
#define BAR 1
#endif

int main()
{
    return BAR;
}

Therefore, if BAR was defined as 2 in foo.h, then the program would return 2 if foo.h exists and 1 if foo.h does not exist.