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.

autogen.sh

@@ -30,8 +30,12 @@ set -ex
 
 # The absence of a m4 directory in googletest causes autoreconf to fail when
 # building under the CentOS docker image. It's a warning in regular build on
-# Ubuntu/gLinux as well.
+# Ubuntu/gLinux as well. (This is only needed if git submodules have been
+# initialized, which is typically only needed for testing; see the installation
+# instructions for details.)
+if test -d third_party/googletest; then
   mkdir -p third_party/googletest/m4
+fi
 
 # TODO(kenton):  Remove the ",no-obsolete" part and fix the resulting warnings.
 autoreconf -f -i -Wall,no-obsolete