Fix atomicops build failure on non-Clang

[edmonds]

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

[edmonds]

BTW, I've tested that this compiles on 8 different Debian platforms: {gcc,clang}/{amd64,i386,powerpc,ppc64el}.

[xfxyjwf]

Can you make this macro only contain the string literal? I remember "#define XXX #error" doesn't compile on some platforms (likely to be VC on windows).

[edmonds]

I copied the style from https://github.com/google/protobuf/blob/v2.6.0/src/google/protobuf/stubs/atomicops.h#L163, so there is already an instance of that construct in the code base :-)

[edmonds]

Also, I'm not quite sure what you're asking for. Do you want the end result to be a bare string constant, causing a syntax error, rather than using #error at all?

[xfxyjwf]

I was suggesting:
#define PLATFORM_ERROR "Host platform ..."
and then
#error PLATFORM_ERROR

[xfxyjwf]

It should be ok if such "#define XXX #error" is already used elsewhere in protobuf. I'll merge the patch.