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/nameidmapping.h

@@ -80,6 +80,7 @@ public:
 		return true;
 	}
 	u16 size() const { return m_id_to_name.size(); }
+
 private:
 	UNORDERED_MAP<u16, std::string> m_id_to_name;
 	UNORDERED_MAP<std::string, u16> m_name_to_id;