Failure when header is included twice with different macros defined

file1.c:
```
# define Y
# include "test.h"

# define BBB
# include "test.h"
```

test.h:
```
#if defined AAA
#elif defined BBB
# undef BBB
#endif

#ifdef BBB
# error adf
#endif
```

Output:
```
$ ~/simplecpp/simplecpp file.c
test.h:8: #error: #error adf
```
