Trivial build system fixes

• subprojects: Make reuse lint happy

• build-aux: Separate downloading dependencies from meson setup

  Downloading dependencies is much slower and needs to be done less often.

• build-aux: Run meson setup with --wipe

  Otherwise, changes to default_options for subprojects will not be taken into account.

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Merging unreviewed because trivial.

/cc @denittis