Have Clang builders build in C++11 mode.

gyp/libwebp.gyp

Index: gyp/libwebp.gyp
diff --git a/gyp/libwebp.gyp b/gyp/libwebp.gyp
index 22a51ba13f42b3e17ab2e48e1159b2a9b70557b5..64ad971798b538e83c4b37be2fa1e660de90697b 100644
--- a/gyp/libwebp.gyp
+++ b/gyp/libwebp.gyp
@@ -4,6 +4,7 @@
 
 {
   'variables': {
+    'skia_warnings_as_errors': 0,
     'conditions':[
       ['skia_android_framework == 1', {
         'use_system_libwebp': 1,
@@ -159,6 +160,9 @@
             'include_dirs': [
               '../third_party/externals/libwebp/src',
             ],
+            'cflags': [
+              '-Wno-unused-function',  # In C++11 mode, we get this warning when including decode.h.
+            ]
           },
           'conditions': [
             ['OS!="win"', {'product_name': 'webp'}],