Disable the build on XCode < 5

Chromium itself requires XCode 5 to build and would require
a few modifications to build with the version of libc++ that
ships with XCode 4.

Disable the build on that environment to at least get passed
the Qt5 CI.

Change-Id: I296055ac1880a9e09e2c0e8343058ff5add837c0
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>

tools/qmake/mkspecs/features/functions.prf

 defineTest(isPlatformSupported) {
   static: return(false)
+  osx:lessThan(QMAKE_XCODE_VERSION, 5): return(false)
+
   linux-g++|win32-msvc2013|macx-clang: return(true)
   return(false)
 }