Fixed Clang 5.0 compile errors (#4085)

* Fixed Clang 5.0 compile errors

* Fixed wrong comment

* Only disable warnings in Clang 5 or higher

* Added a CMake condition for the Clang 5 no-zero-as-null-pointer-constant warning

* Now using the use_nullptr branch of the Cuberite specific SQLiteCpp fork

src/OSSupport/NetworkLookup.h

@@ -20,7 +20,7 @@ class cNetworkLookup :
 public:
 
 	cNetworkLookup();
-	~cNetworkLookup();
+	~cNetworkLookup() override;
 
 	/** Schedule a lookup task for execution. */
 	void ScheduleLookup(std::function<void()> a_Lookup);