Refactor how clang warning flags are set.

third_party/libxslt/libxslt.gyp

Index: third_party/libxslt/libxslt.gyp
diff --git a/third_party/libxslt/libxslt.gyp b/third_party/libxslt/libxslt.gyp
index dea6ae83f0e864198d053e6be7138d10ecf9bf55..55525190cb0550fc6791b7e8aba6ab0fe56448d8 100644
--- a/third_party/libxslt/libxslt.gyp
+++ b/third_party/libxslt/libxslt.gyp
@@ -106,19 +106,14 @@
               '.',
             ],
           },
+          'variables': {
+            'clang_warning_flags': [
+              # libxslt stores a char[3] in a `const unsigned char*`.
+              '-Wno-pointer-sign',
+            ],
+          },
           'conditions': [
             ['OS!="win"', {'product_name': 'xslt'}],
-            ['clang == 1', {
-              'xcode_settings': {
-                'WARNING_CFLAGS': [
-                  # libxslt stores a char[3] in a `const unsigned char*`.
-                  '-Wno-pointer-sign',
-                ],
-              },
-              'cflags': [
-                '-Wno-pointer-sign',
-              ],
-            }],
           ],
         }],
       ],