build: set pkgconfig/dependency variables for features

This allows libseat users to e.g. advise people to chmod a+s the
executable if libseat is built with the builtin backend.

While bumping the Meson version, adjust the scdoc logic to avoid
the following warnings:

    WARNING: Project targeting '>=0.56.0' but tried to use feature deprecated since '0.56.0': Dependency.get_pkgconfig_variable. use Dependency.get_variable(pkgconfig : ...) instead
    WARNING: Project targeting '>=0.56.0' but tried to use feature deprecated since '0.55.0': ExternalProgram.path. use ExternalProgram.full_path() instead

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