Pull define for version out into v8-version-string.h and separate build target

include/v8-version-string.h

+// Copyright 2017 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_VERSION_STRING_H_
+#define V8_VERSION_STRING_H_
+
+#include "include/v8-version.h"
+
+// This is here rather than v8-version.h to keep that file simple and
+// machine-processable.
+
+#if V8_IS_CANDIDATE_VERSION
+#define V8_CANDIDATE_STRING " (candidate)"
+#else
+#define V8_CANDIDATE_STRING ""
+#endif
+
+#define V8_SX(x) #x
+#define V8_S(x) V8_SX(x)
+
+#if V8_PATCH_LEVEL > 0
+#define V8_VERSION_STRING                                                    \
+  V8_S(V8_MAJOR_VERSION)                                                     \
+  "." V8_S(V8_MINOR_VERSION) "." V8_S(V8_BUILD_NUMBER) "." S(V8_PATCH_LEVEL) \
+      V8_CANDIDATE_STRING
+#else
+#define V8_VERSION_STRING \
+  V8_S(V8_MAJOR_VERSION)  \
+  "." V8_S(V8_MINOR_VERSION) "." V8_S(V8_BUILD_NUMBER) V8_CANDIDATE_STRING
+#endif
+
+#endif  // V8_VERSION_STRING_H_