Avoid redundent redefinition of __STDC_FORMAT_MACROS when building with clang

src/arm64/constants-arm64.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_ARM64_CONSTANTS_ARM64_H_
 #define V8_ARM64_CONSTANTS_ARM64_H_
 
 
 // Assert that this is an LP64 system.
 STATIC_ASSERT(sizeof(int) == sizeof(int32_t));     // NOLINT(runtime/sizeof)
 STATIC_ASSERT(sizeof(long) == sizeof(int64_t));    // NOLINT(runtime/int)
 STATIC_ASSERT(sizeof(void *) == sizeof(int64_t));  // NOLINT(runtime/sizeof)
 STATIC_ASSERT(sizeof(1) == sizeof(int32_t));       // NOLINT(runtime/sizeof)
 STATIC_ASSERT(sizeof(1L) == sizeof(int64_t));      // NOLINT(runtime/sizeof)
 
 
 // Get the standard printf format macros for C99 stdint types.
+#ifndef __STDC_FORMAT_MACROS
 #define __STDC_FORMAT_MACROS
+#endif
 #include <inttypes.h>
 
 
 namespace v8 {
 namespace internal {
 
 
 const unsigned kInstructionSize = 4;
 const unsigned kInstructionSizeLog2 = 2;
 const unsigned kLoadLiteralScaleLog2 = 2;
@@ skipping 1212 matching lines @@
 };
 
 enum UnallocatedOp {
   UnallocatedFixed = 0x00000000,
   UnallocatedFMask = 0x00000000
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_ARM64_CONSTANTS_ARM64_H_