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/Root.cpp

@@ -72,7 +72,7 @@ cRoot::cRoot(void) :
 
 cRoot::~cRoot()
 {
-	s_Root = 0;
+	s_Root = nullptr;
 }