Replace override declarations with "virtual" so that we build on Ubuntu 12.04 .

Fixes #1336.

--
MOS_MIGRATED_REVID=124231262
diff --git a/src/main/cpp/blaze.cc b/src/main/cpp/blaze.cc
index a7dce22..384f9ba 100644
--- a/src/main/cpp/blaze.cc
+++ b/src/main/cpp/blaze.cc
@@ -247,11 +247,11 @@
   AfUnixBlazeServer();
   virtual ~AfUnixBlazeServer() {}
 
-  bool Connect() override;
-  void Disconnect() override;
-  unsigned int Communicate() override;
-  void KillRunningServer() override;
-  void Cancel() override;
+  virtual bool Connect();
+  virtual void Disconnect();
+  virtual unsigned int Communicate();
+  virtual void KillRunningServer();
+  virtual void Cancel();
 
  private:
   int server_socket_;
@@ -264,11 +264,11 @@
   GrpcBlazeServer();
   virtual ~GrpcBlazeServer();
 
-  bool Connect() override;
-  void Disconnect() override;
-  unsigned int Communicate() override;
-  void KillRunningServer() override;
-  void Cancel() override;
+  virtual bool Connect();
+  virtual void Disconnect();
+  virtual unsigned int Communicate();
+  virtual void KillRunningServer();
+  virtual void Cancel();
 
  private:
   enum CancelThreadAction { NOTHING, JOIN, CANCEL, COMMAND_ID_RECEIVED };
diff --git a/src/main/cpp/blaze_util_mingw.cc b/src/main/cpp/blaze_util_mingw.cc
index ce0b684..fbcbb4e 100644
--- a/src/main/cpp/blaze_util_mingw.cc
+++ b/src/main/cpp/blaze_util_mingw.cc
@@ -295,7 +295,7 @@
  public:
   DummyBlazeServerStartup() {}
   virtual ~DummyBlazeServerStartup() {}
-  bool IsStillAlive() override { return true; }
+  virtual bool IsStillAlive() { return true; }
 };
 
 void ExecuteDaemon(const string& exe, const std::vector<string>& args_vector,
diff --git a/src/main/cpp/blaze_util_posix.cc b/src/main/cpp/blaze_util_posix.cc
index 112731f..625b291 100644
--- a/src/main/cpp/blaze_util_posix.cc
+++ b/src/main/cpp/blaze_util_posix.cc
@@ -103,7 +103,7 @@
  public:
   PipeBlazeServerStartup(int pipe_fd);
   virtual ~PipeBlazeServerStartup();
-  bool IsStillAlive() override;
+  virtual bool IsStillAlive();
 
  private:
   int pipe_fd;