The Makefile appears to make the erroneous assumption that SDL_mng's headers is going to be in the same place as all the other SDL headers.
On a Debian system, it likely isn't, because Debian package SDL2 but don't package SDL_mng, so if you install SDL_mng from source using the default settings, SDL_mng will be in /usr/local/lib while SDL2 is in of /usr/lib.
The Makefile appears to make the erroneous assumption that SDL_mng's headers is going to be in the same place as all the other SDL headers.
On a Debian system, it likely isn't, because Debian package SDL2 but don't package SDL_mng, so if you install SDL_mng from source using the default settings, SDL_mng will be in /usr/local/lib while SDL2 is in of /usr/lib.