Set Mac OS X deployment target to 10.9

Without it set, the build will choose the current OS version as the deployment target. 10.9 is the earliest usable, since that is when OS X introduced libc++.

platforms/osx/build_framework.py

@@ -18,6 +18,7 @@ class OSXBuilder(Builder):
     def getBuildCommand(self, archs, target):
         buildcmd = [
             "xcodebuild",
+            "MACOSX_DEPLOYMENT_TARGET=10.9",
             "ARCHS=%s" % archs[0],
             "-sdk", target.lower(),
             "-configuration", "Release",