google/protobuf — pull request #27 by edmonds (2014-09-18 23:43:21+00:00)
Title: Fix atomicops build failure on non-Clang

We cannot use Clang's `__has_extension` macro unless we really are compiling on Clang, which means we cannot use this expression:

```
#if (defined(__clang__) && __has_extension(c_atomic)))
// ...
#endif
```

On GCC, this generates the following errors:

```
In file included from ./google/protobuf/stubs/atomicops.h:59:0,
from google/protobuf/stubs/atomicops_internals_x86_gcc.cc:36:
./google/protobuf/stubs/platform_macros.h:67:41: error: missing binary operator before token "("
(defined(__clang__) && __has_extension(c_atomic)))
^
In file included from google/protobuf/stubs/atomicops_internals_x86_gcc.cc:36:0:
./google/protobuf/stubs/atomicops.h:196:40: error: missing binary operator before token "("
(defined(__clang__) && __has_extension(c_atomic))
^
```

Instead, we have to protect the `__has_extension` expression by only executing it when `__clang__` is defined:

```
#if defined(__clang__)
# if __has_extension(c_atomic)
// ...
# endif
#endif
```