[build] Introduce an embedder version string

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 2347be6be86af5af240b6f0f2d72ca3e4f03ff55..d6e5d94e83b9fe8cde8c4c1bb2dacd4108db40fb 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -29,6 +29,9 @@ declare_args() {
   # Enable compiler warnings when using V8_DEPRECATED apis.
   v8_deprecation_warnings = false
 
+  # Allow the embedder to add a custom suffix to the version string.
+  v8_embedder_string = ""
+
   # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
   v8_imminent_deprecation_warnings = ""
 
@@ -174,6 +177,10 @@ config("features") {
 
   defines = []
 
+  if (v8_embedder_string != "") {
+    defines += [ "V8_EMBEDDER_STRING=\"$v8_embedder_string\"" ]
+  }
+
   if (v8_enable_disassembler) {
     defines += [ "ENABLE_DISASSEMBLER" ]
   }