I am thinking this is bad news
If you want to compile bleeding edge Mesa - you need to switch to unstable - and do a full system sync.
sudo pacman-mirrors -aS unstable && sudo pacman -Syyu
Ensure you have package base-devel installed - assuming you did the above full sync
sudo pacman -S base-devel
Clone https://aur.archlinux.org/mesa-git
git clone https://aur.archlinux.org/mesa-git.git
Then build
cd mesa-git && makepkg -cCisr
Remember - you may need to rebuild when dependencies change