Raise minimum OS X requirements

Chromium doesn't build out of the box with older XCode any more.

Change-Id: Ib7241c8768a5fafcdaf4f9b63651028446749c9d
Reviewed-by: Florian Bruhin <qt-project.org@the-compiler.org>
Reviewed-by: Michael Brüning <michael.bruning@qt.io>

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

@@ -114,7 +114,7 @@
 
     \section2 OS X
 
-    On OS X, Xcode version 5.1 or later on OS X 10.9 or later is required.
+    On OS X, Xcode version 6.3 or later on OS X 10.10 or later is required.
 
     \note Qt WebEngine cannot be built for the 32-bit mode of OS X (using the
     \c macx-clang-32 \c mkspec).

tools/qmake/mkspecs/features/functions.prf

@@ -27,14 +27,14 @@ defineTest(isPlatformSupported) {
       return(false)
     }
   } else:osx {
-    lessThan(QMAKE_XCODE_VERSION, 5.1) {
-      skipBuild("Using XCode version $$QMAKE_XCODE_VERSION, but at least version 5.1 is required to build Qt WebEngine.")
+    lessThan(QMAKE_XCODE_VERSION, 6.3) {
+      skipBuild("Using XCode version $$QMAKE_XCODE_VERSION, but at least version 6.3 is required to build Qt WebEngine.")
       return(false)
     }
-    # We require OS X 10.9 (darwin version 13.0.0) or newer
+    # We require OS X 10.10 (darwin version 14.0.0) or newer
     darwin_major_version = $$section(QMAKE_HOST.version, ., 0, 0)
-    lessThan(darwin_major_version, 13) {
-      skipBuild("Qt WebEngine requires OS X version 10.9 or newer.")
+    lessThan(darwin_major_version, 14) {
+      skipBuild("Qt WebEngine requires OS X version 10.10 or newer.")
       return(false)
     }
     !isMinOSXSDKVersion(10, 10, 3): {