Guard against accidental introduction of new Boost dependencies.
Context: #13383 – the usage of boost::lexical_cast was introduced in #11517 from December 2017
utACK e4191d3419ee4ccda0a611632a859e72c40e7ca4
Concept ACK.
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@DrahtBot I'll simply rebase once one of them is merged :-)
utACK 81bbd32a2c755482c6e8ef049a59de672715b545
Concept ACK