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

gyp/common_conditions.gypi

Index: gyp/common_conditions.gypi
diff --git a/gyp/common_conditions.gypi b/gyp/common_conditions.gypi
index f3caddccfedc77b5fc83fd44d4f158ba7710a507..4db0a40358f7cd09fcd788486c695107f7e4fb73 100644
--- a/gyp/common_conditions.gypi
+++ b/gyp/common_conditions.gypi
@@ -348,7 +348,13 @@
         },
         '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',