This makes sure that `AC_USE_SYSTEM_EXTENSIONS` (which
implies `AC_PROG_CC`) is called after the
`AC_ARG_WITH([cc],,)` invocation, so that the proper
CC setting is in scope. Otherwise this can break cross-compilation.

This also needs to pull in a submodule update for `unix`

This is a follow-up commit to
7af29da0
which hopefully fixes #11168