Minetest for C++11 (CMakeLists + Travis)

* Move GCC to GCC 6 & GCC 7
* Move Clang to Clang 3.6 & Clang 4.0
* LINT moves from Clang 3.9 to Clang 4.0
* Move XCode 7.3 to 8.0
* Use more travis tricks to install compilers instead of adding complexity to our build script
* Clang format fixes on checked files (compat Cpp11 instead of Cpp03)
* Mingw GCC update from 4.8.4 to 5.3 (Ubuntu Xenial)
* Drop mingw cmake generated files and add them to gitignore

src/script/lua_api/l_metadata.h

@@ -32,6 +32,7 @@ class MetaDataRef : public ModApiBase
 {
 public:
 	virtual ~MetaDataRef() {}
+
 protected:
 	static MetaDataRef *checkobject(lua_State *L, int narg);