Support pdfium compilation with xcode 10.11 and higher

build/standalone.gypi

Index: build/standalone.gypi
diff --git a/build/standalone.gypi b/build/standalone.gypi
index f118e3003ecf07be607c3a96edd8606bd4ef2a66..c02eaa941492dc9388f5ae36fc38516f1e05665c 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -414,6 +414,15 @@
             '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))',

[Nico, 2016/02/23 19:04:52]

this will silently use the newest sdk on the bots; is that what we want? i think
in chromium land we use find_sdk on developer machines but force a known result
on bots

[Wei Li, 2016/02/23 20:00:35]

Will bot use the same file as the mac_find_sdk in this CL? mac_find_sdk find the
oldest/smallest version which is equal or larger than the minimal sdk version.
In our case, if we have 10.10, we would always go with 10.10. On some machines
which don't have 10.10, we may go up to 10.11 or higher.  

+        },
+        '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': [
@@ -430,11 +439,6 @@
       }],
     ],
   },
-  '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': [