Buck not isolating the compiler process

The compiler process seems to be able to find headers that are not listed as dependencies. Is this expected?

My test case is thus:

```cpp
// hello.cpp
#include
#include "bad.h"

int main() {
printf("Hello World!\n");
return 0;
}

// bad.h

// A commented-out syntax error!
```

And a simple BUCK build script:
```python
cxx_binary(
name="hello",
srcs=["hello.cpp"],
link_style='static'
)
```

As you can see, I have forgotten to list `bad.h` as a dependency.

buck2 will happily compile this and if `bad.h` is changed, it will not recompile anything.