Packer is a wrapper for Arch linux's package manager Pacman and Arch User Repository (AUR). Pacman can not install packages from AUR. This tool helps you do just that.
:$ sudo pacman -S base-devel fakeroot jshon expac git
:$ wget https://aur.archlinux.org/cgit/aur.git/plain/PKGBUILD?h=packer
Rename the downloaded file.
:$ mv PKGBUILD\?h\=packer PKGBUILD
Run makepkg on the PKGBUILD