[ccan] ccan/opt license from GPL3+ to GPL2+?

[Rusty Russell]

Hi all,

        I was looking at cgminer, who use an old version of ccan/opt
which I relicensed to GPLv2+ for them.  They have done some windows
build fixes, so I want to merge it.

Please ack if you're happy to change the license from GPL3 to GPL2 (or
later).

Thanks,
Rusty.