GCC says there are several class types such as DefaultIOSystem, std::unique_ptr that are undeclared. I moved several includes to this header and also added <string> and <assimp/types> because there are strings and Assimp types defined further down.
Added code to use Half-Life 1 MDL loader in MDLLoader.cpp.
Added Half-Life 1 MDL loader files to CMakeLists.
Added new options in config.h to use with Half-Life 1 MDL loader.