NOTE: aur exists, to install via yaourt run
yaourt -S openage-git.
This command should provide required packages for Arch Linux installation:
sudo pacman -S --needed python python-pillow python-numpy python-pygments cython libepoxy ttf-dejavu freetype2 fontconfig harfbuzz cmake sdl2 sdl2_image opusfile opus-tools python-pylint qt5-declarative qt5-quickcontrols
If you don't have a compiler installed, you can select between these commands to install it:
sudo pacman -S --needed gccsudo pacman -S --needed clang
You can install both compilers and select the one to be used by ./configure.