I was installing a fairly old piece of software on Arch Linux, and it was failing because I didn’t have xmkmf installed. After some longer Googling than would be expected, I found that xmkmf is part of imake (which was distributed with XFree86, but is no longer used by XOrg).

So, for the benefit of anyone Googling for, say, “xmkmf: command not found” on an Arch Linux system, install the imake package (pacman -S imake) and all will be well.