*sigh*... unplugged all of my hard drives and plugged them back in again. Now it works.
I figured it out. I got lazy waiting for stuff to compile, so I installed the binaries from the unofficial mingw-w64 repo. I guess they have an older libjpeg version! Rebuilding all the AUR packages myself fixed it.
Does xrandr show the second monitor? If it doesn't, it might be tricky to get it working. If it does show it, it's probably just a matter of setting up your xorg.conf correctly. You can play around with xrandr settings until you find some that work.
I'm trying to install Arch on an Asus Zenbook Prime UX31A. It already has Windows 7 x64 on it and a UEFI system partition doing the bootloading.I followed the steps for creating a UEFI bootable USB from the ISO.