Also cache on appveyor to speed up builds.

* 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.