Run "make doxygen" in a travis builder

[nickm]

With #32385, we now have --enable-fatal-warnings behave sensibly with doxygen warnings. We can prevent ourselves from introducing new doxygen warnings by having a travis builder that does "make doxygen".

This won't apply to warnings about missing documentation: those are off by default, since we have a few thousand of them.

[rl1987]

​https://github.com/torproject/tor/pull/1680

Example run (on unsquashed branch): ​https://travis-ci.org/rl1987/tor/jobs/640408146?utm_medium=notification&utm_source=github_status

[nickm]

I wonder if we can add this to an existing builder, instead of making a new builder? Using a separate builder here makes us recompile Tor unnecessarily, which I would expect would slow down CI.

[nickm]

Setting owner.

Rl1987, please let us know if you won't be revising this in the next month or so? I'd like to get it into 0.4.3 if possible.

[teor]

Replying to nickm:

I wonder if we can add this to an existing builder, instead of making a new builder? Using a separate builder here makes us recompile Tor unnecessarily, which I would expect would slow down CI.

I suggest the coverage build, in case we ever want to do documentation coverage.

We could also add a new builder, but run "make doxygen" instead of "make" or "make src/app/tor". Linux builders only cost us a few 10s of seconds to start up and clone tor.

[rl1987]

But it does not recompile Tor binary (SKIP_MAKE_CHECK is set to true)? Above example build took 1 min. 33 seconds to complete.

I can move make doxygen command to the end of coverage build though. This would be slightly faster, as we wouldn't need to rerun configure.

[nickm]

Oh, you're right! I had thought incorrectly that there was a make all invocation that SKIP_MAKE_CHECK didn't skip. I think this patch is fine, then. Merged, and thanks for your patience!