this might be a little more makefile-ish:

mig-agent-all: mig-agent-i386 mig-agent-x86_64

mig-agent-i386:
   OS=... ARCH=... make mig-agent

mig-agent-x86_64:
  OS=... ARCH... make mig-agent

it will also stop on failure, but allow for building arch independently, and is more readable, IMO