Rob Vermaas and Ludovic Courtès took the initiative to set up a
continuous build for Emacs trunk on Hydra at
http://hydra.nixos.org/jobset/gnu/emacs-trunk .
It builds (and runs ERT tests) on these platforms:
i686-cygwin
i686-darwin
i686-freebsd
i686-linux
x86_64-darwin
x86_64-linux
The first three of these are currently reported as broken; I presume
this is not because Emacs is actually broken on these platforms, but
because Emacs' build recipe on Hydra needs further tweaking. I will
look into this later next week (unless someone beats me to it).