Remove legacy dependencies and fix aarch64 build

The mixxx package currently does not build on aarch64, due to some missing legacy dependencies and a GCC bug. As it turns out, these are not actually needed and the GCC bug can easily be worked around by making -Werror slightly less restrictive. More info on this in the commit messages.

I have submitted these patches to the Arch Linux ARM repo, but given that the ALARM maintainers seem to be pretty busy and that these changes are rather generic, it might be nice to have them upstream anyway, so downstream distros can just build the package without modifications.