CTFE and BetterC compatibility

[rikki cattermole]

This works:

```d
struct Data
{
     int[] digits;
}

int parseDigit(char c) pure
{
     return c - '0';
}

Data parse(string str) pure
{
     Data data;

     if (__ctfe) {
         size_t used;
         data.digits.length = str.length;

         while (str.length != 0)
         {
             // Skip spaces
             while (str[0] == ' ')
                 str = str[1 .. $];

             // Parse single digit integer
             data.digits[used++] = parseDigit(str[0]);

             // Consume digit
             str = str[1 .. $];
         }

         data.digits = data.digits[0 .. used];
     }

     return data;
}

enum Data parsedData = parse("5 4 2 6 9");

extern(C) int main()
{
     pragma(msg, "First digit=", parsedData.digits[0]);
     return 0;
}
```