Allow an override of MACOSX_DEPLOYMENT_TARGET, so the user can built against a different SDK if des…

gyp/common_conditions.gypi

 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
     'SK_SUPPORT_GPU=<(skia_gpu)',
     'SK_SUPPORT_OPENCL=<(skia_opencl)',
     'SK_DISTANCEFIELD_FONTS=<(skia_distancefield_fonts)',
   ],
   'conditions' : [
@@ skipping 330 matching lines @@
           },
           'Release': {
             'xcode_settings': {
               'GCC_OPTIMIZATION_LEVEL': '<(skia_release_optimization_level)',
             },
             'defines': [ 'NDEBUG' ],
           },
         },
         'xcode_settings': {
           'GCC_SYMBOLS_PRIVATE_EXTERN': 'NO',
-          'MACOSX_DEPLOYMENT_TARGET': '10.6', # -mmacos-version-min, passed in environment to ld.
+          'conditions': [
+            [ 'skia_osx_deployment_target==""', {
+              'MACOSX_DEPLOYMENT_TARGET': '10.6', # -mmacos-version-min, passed in environment to ld.
+            }, {
+              'MACOSX_DEPLOYMENT_TARGET': '<(skia_osx_deployment_target)',
+            }],
+          ],
 # trying to get this to work, but it needs clang I think...
 #          'WARNING_CFLAGS': '-Wexit-time-destructors',
           'CLANG_WARN_CXX0X_EXTENSIONS': 'NO',
           'GCC_WARN_64_TO_32_BIT_CONVERSION': 'YES',
           'GCC_WARN_ABOUT_DEPRECATED_FUNCTIONS': 'YES',
           'GCC_WARN_ABOUT_INVALID_OFFSETOF_MACRO': 'YES',
           'GCC_WARN_ABOUT_MISSING_NEWLINE': 'YES',
           'GCC_WARN_ABOUT_MISSING_PROTOTYPES': 'YES',
           'GCC_WARN_ABOUT_POINTER_SIGNEDNESS': 'YES',
           'GCC_WARN_ABOUT_RETURN_TYPE': 'YES',
@@ skipping 106 matching lines @@
       'defines': [
         'SK_USE_POSIX_THREADS',
       ],
     }],
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }