Allow root build dependencies to be overridden.

R=andrew@webrtc.org, thorcarpenter@google.com

Review URL: https://webrtc-codereview.appspot.com/22039004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6856 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/all.gyp b/all.gyp
index 4830c54..b43c3a1 100644
--- a/all.gyp
+++ b/all.gyp
@@ -10,6 +10,8 @@
   'variables': {
     'libjingle_root%': '<(DEPTH)',
     'include_tests%': 1,
+    'webrtc_root_additional_dependencies': [
+    ],
   },
   'targets': [
     {
@@ -20,6 +22,7 @@
         'webrtc/webrtc.gyp:*',
         '<(libjingle_root)/talk/libjingle.gyp:*',
         '<(libjingle_root)/talk/libjingle_examples.gyp:*',
+        '<@(webrtc_root_additional_dependencies)',
       ],
       'conditions': [
         ['OS=="android"', {