Check that C++ compiler supports override keyword

If the compiler supports override, define CXX11_OVERRIDE as override,
otherwise define it as empty. Use CXX11_OVERRIDE instead of override.

test/GZipDecodingStreamFilterTest.cc

@@ -30,12 +30,12 @@ class GZipDecodingStreamFilterTest:public CppUnit::TestFixture {
   public:
     MockSegment2():positionToWrite_(0) {}
 
-    virtual void updateWrittenLength(int32_t bytes) override
+    virtual void updateWrittenLength(int32_t bytes) CXX11_OVERRIDE
     {
       positionToWrite_ += bytes;
     }
 
-    virtual int64_t getPositionToWrite() const override
+    virtual int64_t getPositionToWrite() const CXX11_OVERRIDE
     {
       return positionToWrite_;
     }