Incorrect expansion of functional macros

Header:
```c
#define B A
#define A x(B)
#define C(s) s
#define D(s) C(s)

D(A)
```

gcc output:
```c
x(A)
```

simplecpp output:
```c
x ( x ( A ) )
```

I couldn't find an earlier commit where this isn't the output. I believe this issue is related to #549.