Refactor how clang warning flags are set.

third_party/libxml/libxml.gyp

Index: third_party/libxml/libxml.gyp
diff --git a/third_party/libxml/libxml.gyp b/third_party/libxml/libxml.gyp
index c005f6a89c6790666b596d12fffb8137ab965162..d6097786b4e3ee08ae132b57fd9495913b473e88 100644
--- a/third_party/libxml/libxml.gyp
+++ b/third_party/libxml/libxml.gyp
@@ -188,6 +188,21 @@
             # defines the macro FOO as 1.)
             'LIBXML_STATIC=',
           ],
+          'variables': {
+            'clang_warning_flags': [
+              # libxml passes `const unsigned char*` through `const char*`.
+              '-Wno-pointer-sign',
+              # pattern.c and uri.c both have an intentional
+              # `for (...);` / `while(...);` loop. I submitted a patch to
+              # move the `'` to its own line, but until that's landed
+              # suppress the warning:
+              '-Wno-empty-body',
+              # debugXML.c compares array 'arg' to NULL.
+              '-Wno-tautological-pointer-compare',
+              # See http://crbug.com/138571#c8
+              '-Wno-ignored-attributes',
+            ],
+          },
           'include_dirs': [
             '<(os_include)',
             '<(os_include)/include',
@@ -234,27 +249,6 @@
               'product_name': 'xml2',
             }],
             ['clang==1', {
-              'xcode_settings': {
-                'WARNING_CFLAGS': [
-                  # libxml passes `const unsigned char*` through `const char*`.
-                  '-Wno-pointer-sign',
-                  # pattern.c and uri.c both have an intentional
-                  # `for (...);` / `while(...);` loop. I submitted a patch to
-                  # move the `'` to its own line, but until that's landed
-                  # suppress the warning:
-                  '-Wno-empty-body',
-                  # debugXML.c compares array 'arg' to NULL.
-                  '-Wno-tautological-pointer-compare',
-                ],
-              },
-              'cflags': [
-                '-Wno-pointer-sign',
-                '-Wno-empty-body',
-                '-Wno-tautological-pointer-compare',
-
-                # See http://crbug.com/138571#c8
-                '-Wno-ignored-attributes',
-              ],
               'msvs_settings': {
                 'VCCLCompilerTool': {
                   'AdditionalOptions': [