The Arch package repositories haven't updated to the latest Haskell Platform yet, and I needed the absolute latest version (for GHC 7.0.3). I went to download the platform and a ghc binary to install it manually. Trouble struck when I tried to ./configure ghc.

I get some message about not being able to find libgmp.so.3. How annoying.

Long story short, go download GMP 4 (a legacy version which provides libgmp.so.3) here: https://aur.archlinux.org/packages.php?ID=36512. Install that (unpack it, enter directory, makepkg -s, pacman -U *.xz), and continue on your merry way.