Support pdfium compilation with xcode 10.11 and higher

build/standalone.gypi

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Definitions to be used when building stand-alone PDFium binaries.
 
 {
   'variables': {
     'component%': 'static_library',
     'clang%': 0,
@@ skipping 398 matching lines @@
             },
           }],
         ],
       }],  # OS=="win"
       ['OS=="mac"', {
         'target_conditions': [
           ['_type!="static_library"', {
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-search_paths_first']},
           }],
         ],  # target_conditions
+        'variables': {
+          'mac_sdk_min': '10.10',
+          'mac_sdk%': '<!(python <(DEPTH)/build/gyp/tools/mac_find_sdk.py <(mac_sdk_min))',
+        },
+        'xcode_settings': {
+          'SDKROOT': 'macosx<(mac_sdk)',  # -isysroot
+          # See comment in Chromium's common.gypi for why this is needed.
+          'SYMROOT': '<(DEPTH)/xcodebuild',
+        },
       }],  # OS=="mac"
       ['v8_use_external_startup_data==1', {
         'defines': [
           'V8_USE_EXTERNAL_STARTUP_DATA',
         ],
       }],  # v8_use_external_startup_data==1
       ['clang==1 or host_clang==1', {
         # This is here so that all files get recompiled after a clang roll and
         # when turning clang on or off.
         # (defines are passed via the command line, and build systems rebuild
         # things when their commandline changes). Nothing should ever read this
         # define.
         'defines': ['CR_CLANG_REVISION=<!(python <(DEPTH)/tools/clang/scripts/update.py --print-revision)'],
       }],
     ],
   },
-  'xcode_settings': {
-    'SDKROOT': 'macosx10.10',  # -isysroot
-    # See comment in Chromium's common.gypi for why this is needed.
-    'SYMROOT': '<(DEPTH)/xcodebuild',
-  },
   'conditions': [
     ['OS=="linux" or OS=="mac"', {
       'conditions': [
         ['clang==1', {
           'make_global_settings': [
             ['CC', '<(clang_dir)/bin/clang'],
             ['CXX', '<(clang_dir)/bin/clang++'],
           ],
         }],
       ],
     }],  # OS=="linux" or OS=="mac"
     ["use_goma==1", {
       'make_global_settings': [
         ['CC_wrapper', '<(gomadir)/gomacc'],
         ['CXX_wrapper', '<(gomadir)/gomacc'],
       ],
     }],  # use_goma==1
   ],
 }