Fix compilation error under Clang 4.0.

The LLVM trunk has reached 4.0 and now __clang_major__ is not enough
to distinguish between old XCode Clang and the new 'real' Clang.
Using __apple_build_version__ allows to make this distinction.

BUG=chromium:631144

Change-Id: I0b6e46fddfe4f409c7b7e558bda34872e60ee2d9

vpx_dsp/x86/vpx_subpixel_8t_intrin_avx2.c

@@ -37,7 +37,7 @@ DECLARE_ALIGNED(32, static const uint8_t, filt4_global_avx2[32]) = {
 
 #if defined(__clang__)
 # if __clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ <= 3) || \
-    (defined(__APPLE__) && \
+    (defined(__APPLE__) && defined(__apple_build_version__) && \
         ((__clang_major__ == 4 && __clang_minor__ <= 2) || \
             (__clang_major__ == 5 && __clang_minor__ == 0)))