Lower minimum OS X requirements back to 10.9

This reverts commit c8851dd1

,
and allows building on OS X 10.9 with OS X 10.10 SDK.

Change-Id: Id59b08424165272fd0d35418eef45a03fce731fb
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

src/webengine/doc/src/qtwebengine-platform-notes.qdoc

@@ -104,8 +104,8 @@
     On OS X, the following are required:
 
     \list
-        \li OS X 10.10 or later
-        \li Xcode 6.3 or later
+        \li OS X 10.9 or later
+        \li Xcode 5.1 or later
         \li OS X 10.10 SDK or later
     \endlist
 

tools/qmake/mkspecs/features/functions.prf

@@ -43,14 +43,14 @@ defineTest(isPlatformSupported) {
       return(false)
     }
   } else:osx {
-    lessThan(QMAKE_XCODE_VERSION, 6.3) {
-      skipBuild("Using XCode version $$QMAKE_XCODE_VERSION, but at least version 6.3 is required to build Qt WebEngine.")
+    lessThan(QMAKE_XCODE_VERSION, 5.1) {
+      skipBuild("Using XCode version $$QMAKE_XCODE_VERSION, but at least version 5.1 is required to build Qt WebEngine.")
       return(false)
     }
-    # We require OS X 10.10 (darwin version 14.0.0) or newer
+    # We require OS X 10.9 (darwin version 13.0.0) or newer
     darwin_major_version = $$section(QMAKE_HOST.version, ., 0, 0)
-    lessThan(darwin_major_version, 14) {
-      skipBuild("Qt WebEngine requires OS X version 10.10 or newer.")
+    lessThan(darwin_major_version, 13) {
+      skipBuild("Qt WebEngine requires OS X version 10.9 or newer.")
       return(false)
     }
     !isMinOSXSDKVersion(10, 10): {