Add a way to easily test that makedoc.g works

[fingolfin]

Clearly the documentation should always be buildable, but sometimes people break it anyway; of course when making a release, this becomes evident, but it is desirable to notice this immediately.

Hence it would be good to test that, at least for packages using makedoc.g. It is potentially problematic to just always do that (as running makedoc.g might have additional effects for some packages, and thus cause issues); also, if a package's makedoc.g requires a recent AutoDoc version, it may e.g. fail in the stable-4.9 branch.

I see two ways to proceed:

1. Opt-In: we add a way for a package author to specify that makedoc.g is supposed to be tested in one specific build, probably by checking for a new env var, like TEST_MAKEDOC=yes
2. Opt-out: always test for makedoc.g, unless TEST_MAKEDOC=no is specified (possibly with some restrictions: e.g. we might want to limit this to builds on master)

The advantage of 1 is that it won't unexpectedly break any package's test suite. The drawback is that now every package needs to opt-in to benefit.

[fingolfin]

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