Because operating systems are generally backwards compatible, to build binaries that work on several versions of Linux it's best to build them on the oldest version of the OS that's still supported. For Ubuntu this is currently LTS 12, and for CentOS it's 5. While I can get a VM for either of these on Rackspace etc, at this point I can only get 64-bit versions and some people still need 32-bit binaries. So, here's how to make a 32-bit chroot environment for building them.

Ubuntu

Derived from jblevins.org/log/ubuntu-chroot.

$ sudo apt-get install debootstrap dchroot
$ sudo emacs /etc/schroot/schroot.conf
[precise_i386]
description=Ubuntu 12.04 Precise for i386
directory=/var/chroot/precise_i386
personality=linux32
root-users=buildbot
users=buildbot

$ sudo emacs /etc/schroot/mount-defaults
comment out the /home line (Cross-mounting
/home makes moving files much easier.)
$ sudo debootstrap --variant=buildd --arch i386 precise
/var/chroot/precise_i386 http://archive.ubuntu.com/ubuntu/
$ sudo cp /etc/resolv.conf /var/chroot/precise_i386/etc/resolv.conf
$ sudo cp /etc/apt/sources.list /var/chroot/precise_i386/etc/apt/
$ pushd /var/chroot/precise_i386/etc
$ for x in passwd shadow group gshadow hosts sudoers; do
    sudo rm $x; sudo ln /etc/$x;
done
$ popd
$ sudo emacs /etc/fstab
add:
/dev /var/chroot/precise_i386/dev none bind 0 0
proc-chroot /var/chroot/precise_i386/proc proc defaults 0 0
devpts-chroot /var/chroot/precise_i386/dev/pts devpts defaults 0 0
none /var/chroot/precise_i386/dev/shm tmpfs rw,nosuid,nodev,noexec 0 0
$ sudo mount -a
$ sudo chroot /var/chroot/precise_i386/
# apt-get update
# apt-get install gnupg locales dialog emacs wget
# locale-gen en_US.UTF-8
# dpkg-reconfigure tzdata
# dpkg-reconfigure passwd
# apt-get install sudo
# mkdir /home/buildbot
# chown buildbot /home/buildbot
# exit
$ sudo emacs /var/chroot/precise_i386/etc/debian_chroot
precise_i386
$ schroot -c precise_i386
now you're in the chroot

CentOS

I wish there was debootstrap for CentOS.

$ sudo yum install setarch
$ sudo mkdir -p /var/chroot/centos_i386/var/lib/rpm \
                /var/chroot/centos_i386/home/buildbot
$ sudo cp /etc/rpm/platform  /etc/rpm/platform.real
$ echo i686-redhat-linux | sudo tee /etc/rpm/platform
$ sudo rpm --rebuilddb --root=/var/chroot/centos_i386
$ URL="http://mirror.centos.org/centos/5/os/i386/"
$ URL+="CentOS/centos-release-5-10.el5.centos.i386.rpm"
$ wget "$URL"
$ sudo rpm --root=/var/chroot/centos_i386 --nodeps -i \
       centos-release-5-10.el5.centos.i386.rpm
$ sudo yum --installroot=/var/chroot/centos_i386/ update
$ sudo yum --installroot=/var/chroot/centos_i386/ install \
           -y rpm-build yum
$ sudo mv /etc/rpm/platform.real  /etc/rpm/platform
$ sudo cp /etc/resolv.conf /var/chroot/centos_i386/etc/resolv.conf
$ pushd /var/chroot/centos_i386/etc
$ sudo rm group passwd
$ for x in passwd shadow group gshadow hosts sudoers; do
    sudo ln /etc/$x;
done
$ popd
(All these mounts should really be put into /etc/fstab.)
$ sudo mount -t proc proc /var/chroot/centos_i386/proc
$ sudo mount -t sysfs none /var/chroot/centos_i386/sys
$ sudo mount --bind /dev /var/chroot/centos_i386/dev
$ sudo mount -t tmpfs tmpfs /var/chroot/centos_i386/dev/shm
$ sudo mount --rbind /selinux /var/chroot/centos_i386/selinux
$ sudo mount --rbind /etc/selinux \
       /var/chroot/centos_i386/etc/selinux
$ sudo mount --rbind /home/buildbot \
       /var/chroot/centos_i386/home/buildbot
$ sudo setarch i386 /usr/sbin/chroot \
       /var/chroot/centos_i386/ /bin/bash -l
# rm /var/lib/rpm/__db.00*
now you're in the chroot