Workaround clang4.0 <stdatomic.h> initialization #trivial

[bkase]

Texture fails to build with this error on clang 4.0:

external/Texture/pod_support/Headers/Public/AsyncDisplayKit/ASDispatch.h:32:35: error: illegal initializer type 'atomic_size_t' (aka '_Atomic(size_t)')
  __block atomic_size_t counter = ATOMIC_VAR_INIT(0);
                                  ^
In module 'std' imported from external/Texture/pod_support/Headers/Public/AsyncDisplayKit/ASStackUnpositionedLayout.h:18:
/private/var/tmp/_bazel_bkase/a00d4cbe29902fb63d5778cc19944cd2/external/clang40/bin/../include/c++/v1/atomic:1839:30: note: expanded from macro 'ATOMIC_VAR_INIT'
                             ^
1 error generated.

See
http://techqa.info/programming/question/38233019/Initializing-an--atomic-int--with-a-braced-constant--Is-this-valid-C-code--If-so-why-does-it-not-compile-in-clang-

Replacing the intialization with just 0 (and not the macro fixes the
error). The forum post notes:

C11's <stdatomic.h> header defines a macro ATOMIC_VAR_INIT that's intended to be used to initialize atomic objects, with an example:
atomic_int guide = ATOMIC_VAR_INIT(42);
It appears to be needed for atomic objects with automatic storage duration, which doesn't apply here.

Since we don't need the automatic storage duration either, this code
should still be correct.

[CLAassistant]

All committers have signed the CLA.

[bkase]

@maicki looked at this with me and confirmed this should be okay

[maicki]

Looks good to me, but let's have @Adlai-Holler have another look over it.

[Adlai-Holler]

@bkase We actually do need automatic storage duration on this variable =/.

Since the current implementation of ATOMIC_VAR_INIT simply returns the value unaffected, I'm fine with this patch if it unblocks you, but we should add a comment explaining why we aren't using the API-provided initializer.

[Adlai-Holler]

P.S. A more complete solution to this is to move the implementation of ASDispatchApply into a .m (or .c) file so that it's never compiled by a C++ compiler.

In the header just surround the function declaration by ASDISPLAYNODE_EXTERN_C_BEGIN/END so that C++ can see the function.

[bkase]

@Adlai-Holler yeah it's blocking the bazel build of master -- how's that comment?

[Adlai-Holler]

Down with it