[ccan] Using opt with -Wextra and -Werror

[Joel Stanley]

Hello Rusty,

I wrote the two following patches so that we could use opt in our project that
has -Wextra and -Werror enabled.

No worries if they're not suitable for the main tree, but I thought I'd put
them out there incase others come across similiar issues.  It wasn't obvious
what was going wrong at some points, due to the crazy amount of macro
expansions when looking at the preproceed source.  Using clang instead of gcc
helped alot there.

Cheers,

Joel