On the latest versions of Linux, you should be able to stick the SHMMAX value (in bytes) in /etc/sysctl.conf:
kernel.shmmax = 134217728
On slightly older versions, you should be able to do this:
cat 134217728 > /proc/sys/shmmax
In any case, I recompile the kernel anyhow for other stuff (e.g., increase NR_TASKS, MIN_TASKS_LEFT_FOR_ROOT in include/linux/tasks.h; add ReiserFS and crypto patches; remove RAID support), so tossing in a change to SHMMAX is automatic.