demote missing gtest to a buildtime warning

We just need it for unit tests and our debian/rules file actually skips
calling them if nocheck is given… but this fails anyhow as we declared a
hard-dependency on it. Demoting the error to a warning in configuration
and adding a test in the 'make test' path with a friendly message allows
nocheck to be useful again.

(Running unit tests is fully encouraged of course, but bootstrappers and
 co do not need to be burdened with this stuff)

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