New build dependency on autotools

c-deps: vendor with submodules, not tarballs (#16859), which introduces a compile-time dependency on autotools, is inches from landing! If you don’t already have the autotools suite on your machine, you’ll need to install it to build CockroachDB from source after this PR lands.

With Homebrew:

$ brew install autoconf automake libtool

With apt:

$ apt-get install autoconf automake libtool

Folks who use the builder (build/builder.sh) don’t need to do anything. Folks who use an azworker/gceworker will need to recreate or rebootstrap their worker to pick up the new dependencies, or manually install autotools using the apt instructions above.

Sorry for the inconvenience! This change will make it it far easier to update our C/C++ dependencies, especially the ones we maintain patches for. I’m hoping it addresses some of the build flakiness, too.

Note that thanks to @tamird’s hard work, we’ve eliminated our dependency on automake and libtool. As always, see CONTRIBUTING.md for the most up-to-date list of build dependencies.