C version of #1006
Note that I switched all the C builds to use -O2 and that meant
that the compiler found some more warnings so I also fixed these
up. The possibly undefined ones were harmless, the aliasing one
is now more correct.
As these are small programs, the caching from `docker build` makes
no real difference, and worst case compile time is much better.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
