Adjust max OS versions according to Xcode 11 versions.

Make.config

@@ -90,11 +90,12 @@ OSX_SDK_VERSION=10.15
 WATCH_SDK_VERSION=6.0
 TVOS_SDK_VERSION=13.0
 
-# Temporary max versions because iOS 12.3 and 12.4 are supported by the 12.2 SDK.
-# When iOS 13 comes out we can revert to checking against the SDK version
-MAX_IOS_VERSION=12.4
-MAX_WATCH_VERSION=5.3
-MAX_TVOS_VERSION=12.4
+# If the max OS version does not match the SDK version (for instance the iOS
+# 12.2 SDK supported both iOS 12.3 and iOS 12.4), then these variables can be
+# set to something other than the corresponding SDK versions.
+MAX_IOS_VERSION=$(IOS_SDK_VERSION)
+MAX_WATCH_VERSION=$(WATCH_SDK_VERSION)
+MAX_TVOS_VERSION=$(TVOS_SDK_VERSION)
 
 # Minimum OS versions for running XI/XM apps.
 MIN_IOS_SDK_VERSION=7.0