[build] Introduce an embedder version string

gypfiles/features.gypi

Index: gypfiles/features.gypi
diff --git a/gypfiles/features.gypi b/gypfiles/features.gypi
index f82a59f0f82f53718bae76fc53e248c89f541577..4669fcf8bad8137d1425a2bb46f60d7243e657ef 100644
--- a/gypfiles/features.gypi
+++ b/gypfiles/features.gypi
@@ -64,6 +64,9 @@
     # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
     'v8_imminent_deprecation_warnings%': 0,
 
+    # Allow the embedder to add a custom suffix to the version string.
+    'v8_embedder_string%': '',
+
     # Set to 1 to enable DCHECKs in release builds.
     'dcheck_always_on%': 0,
 
@@ -105,6 +108,9 @@
       ['v8_use_snapshot=="true" and v8_use_external_startup_data==1', {
         'defines': ['V8_USE_EXTERNAL_STARTUP_DATA',],
       }],
+      ['v8_embedder_string!=""', {
+        'defines': ['V8_EMBEDDER_STRING="<(v8_embedder_string)"',],
+      }],
       ['dcheck_always_on!=0', {
         'defines': ['DEBUG',],
       }],