c2hs fails on MacOS

`c2hs` fails with errors like this;

```
Building grpc-0.1.0.0...
Preprocessing library grpc-0.1.0.0...
c2hs: C header contains errors:

/usr/include/time.h:153: (column 72) [ERROR] >>> Syntax error !
The symbol `__attribute__' does not fit here.
```

It's due to that c2hs fails to parse the C headers. We need something like this in the `.cabal` file, or add a new header file;

```diff
+ cpp-options:
+ -D__attribute__(x)=
+ -Drestrict=
+ -D__qos_class=
+ -D__relative_priority=
+ -D_Nullable=
+ -D_Nonnull=
```