I've tested it and run the example here
https://iroha.readthedocs.io/en/latest/getting_started/cli-guide.html
It works!

Patch looks good, feel free to merge.
A few comments below:

> +              (snippet
> +               '(begin
> +                  ;; Backport unversioned fmt dependency:
> +                  (substitute* "libs/logger/logger.hpp"
> +                    (("fmt::v5") "fmt"))
> +                  #t))))

Mention the commit you've sent me and that this should be removed on
next update.

> +    (build-system cmake-build-system)
> +    (arguments
> +     `(#:configure-flags
> +       '("-DTESTING=OFF"
> +         ; Don't install the shared libraries of the dependencies:
> +         "-DENABLE_LIBS_PACKAGING=OFF")
> +       #:tests? #f

Explain why the tests must be disabled.

> +       #:test-target "."

Why setting the test-target if you are not running it?

> +       #:phases
> +       (modify-phases %standard-phases
> +         (add-after 'unpack 'change-compilation-flags
> +           (lambda _
> +             (substitute* "cmake/functions.cmake"
> +               (("-Werror") ""))
> +             #t)))))

Why do you need to do this?

> +    ;; See https://iroha.readthedocs.io/en/latest/build/index.html.
> +    (native-inputs
> +     `(("googletest" ,googletest)))
> +    (inputs
> +     `(("boost" ,boost-1.69)
> +       ("fmt" ,fmt)
> +       ("gflags" ,gflags)
> +       ("grpc" ,grpc)
> +       ("hyperledger-iroha-ed25519" ,hyperledger-iroha-ed25519)
> +       ("postgresql" ,postgresql)
> +       ("protobuf" ,protobuf)
> +       ("rapidjson" ,rapidjson)
> +       ("rxcpp" ,rxcpp)
> +       ("soci" ,soci)
> +       ("spdlog" ,spdlog)
> +       ("tbb" ,tbb)))
> +    (synopsis "Simple, decentralized ledger")
> +    (description "Iroha is a distributed ledger technology (DLT). Iroha has

Double spacing.

Thanks!
Cheers!

-- 
Pierre Neidhardt
https://ambrevar.xyz/