Build updates (#5300)

* Make language instructions' documentation links clickable

See 86208c52.

* Add a `git submodules` check to autogen.sh

Running `./autogen.sh && make` happens to work without initializing git
submodules - but autogen.sh writes to what is supposed to be a git
submodules directory, so trying to later initialize git submodules
fails.

This change makes building a bit more reliable for people who miss this
step.

* Revise third_party googletest logic

As discussed in PR, instead of requiring that `git submodule --init` be
run, correctly handle the case where it isn't run.