Factor out xcode c/cpp settings into variables to apply to cmake builds

gyp/icu.gyp

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'includes': [
     'common_variables.gypi',
   ],
   'variables': {
     'component%': 'static_library',
@@ skipping 102 matching lines @@
         ],
         [
           'skia_os in ["linux", "chromeos"]', {
             'sources': [ '<(icu_directory)/linux/icudtl_dat.S', ],
           }
         ],
         [
           'skia_os == "mac"', {
             'sources': [ '<(icu_directory)/mac/icudtl_dat.S', ],
             'xcode_settings': {
-              'GCC_ENABLE_CPP_RTTI': 'YES',  # -frtti
+              'OTHER_CPLUSPLUSFLAGS': [ '-frtti' ],
               'WARNING_CFLAGS': [ '-w' ],
             },
           }
         ],
       ], # conditions
     },
   ], # targets
 }