* Override the compiler via CC; ./configure can't seem to detect it.
* Unset CROSS_COMPILE when running make. Per the comment, we otherwise
end up invoking commands like i686-ar.
Specifically:
* Update the base docker images to ubuntu 16.10
* Update musl to version 1.1.15
This is necessary since the versions we were previously using do not
define CLONE_NEWCGROUP, so adding the symbol to the libc crate causes
test failures.
