Do you want Super Mario War on your Steam Deck? Well, here's my experiences with it!
NOTE: Do not run commands with the quotation marks, just what is contained within the quotations.
Step 0: Reboot into desktop mode. Open youself up a terminal (default is Konsole).
Step 1: Set a root password for your Steam Deck with the command "passwd".
Step 2: Disable SteamOS's read-only measure by the command "sudo steamos-readonly disable".
Step 3: Install Super Mario War's prerequisites (CMake; Git; SDL; SDL_image; SDL_mixer; Zlib) with the command "sudo pacman -S cmake git sdl sdl_image sdl_mixer zlib".
Step 4: Get the Super Mario War 2.0 source code by the command "git clone https://github.com/mmatyas/supermariowar.git".
Step 5: Change you directory to the downloaded code via "cd supermariowar".
Step 6: Make [and cd into] a build directory for the cmake building. "mkdir build && cd build".
Step 7: Start the cmake build process with "cmake .."
Step 8: Start the make process to compile Super Mario War with the command "make -j4"
Step 9: When the makefile has completed its job, there should be a ./Binaries/Release/ folder ('.' meaning current directory). I usually copy the executables in ./Binaries/Release to /usr/local/bin by "sudo cp -r ./Binaries/Release/* /usr/local/bin".
Step 10: Open Steam, and add a "Non-Steam Game" [in the bottom left corner], or some phrasing of the sort.
Step 11: Navigate to where we copied the executables to (/usr/local/bin), and add the "/usr/local/bin/smw" executable.
Step 12: For safety/security reasons remember to enable SteamOS's read-only measure. "sudo steamos-readonly enable"
Step 13: Make sure to set your controls on your Steam Deck, otherwise, have fun playing with a connected keyboard.
Step 14: You should be all good to play it for now. A SteamOS upgrade *might* remove the Arch packages that Super Mario War depends on. If it does, just run commands from Step 2, 3, and 12 again in that order.