This PR add support for CAPNP_BUILD=autotools-static in ci/scripts/ci.sh so Cap'n Proto can be built from source using autotools instead of cmake. This provides CI coverage for static linking against autotools-built Cap'n Proto libraries and ensures link order regressions are caught.
Enable CAPNP_BUILD=autotools-static in newdeps, and adds autoconf, automake, and libtool to nativeBuildInputs in shell.nix for the nix environment.
As mentioned in #367#pullrequestreview-5380384729.