Add mixxx-git

[fwcd]

Fixes #3

This adds Mixxx, a free and open-source DJ application using a PKGBUILD based on the AUR package.

It seems to work, but is exceptionally slow to build under QEMU/aarch64 (>6h with make, ~5h with ninja), therefore we might want to consider holding off merging this PR before we figure out a solution. Some ideas:

• Exclude mixxx-git from the aarch64 build somehow
• Cross-compile Mixxx to aarch64 Linux separately (e.g. in another repo), then only package the binaries
• Figure out a way to cross-build Arch packages directly
  • Unfortunately, this does not seem to be a well-supported scenario
  • We'd need a cross-toolchain on the host
  • Several potential approaches for actually using it:
    • Run everything on the host, use a custom PKGBUILD that passes the right sysroot to cmake and explicitly override the architecture with CARCH=aarch64 makepkg
    • Use distcc on the host, but run makepkg within QEMU and figure out some way to reference it from within QEMU
  • See also:
    • https://unix.stackexchange.com/questions/80833/cross-compiling-with-makepkg-command
    • https://archlinuxarm.org/forum/viewtopic.php?t=6163
    • https://bbs.archlinux.org/viewtopic.php?id=162429
    • https://archlinuxarm.org/wiki/Distributed_Compiling
    • https://www.reddit.com/r/archlinux/comments/kqa7l2/i_made_arch_linux_for_powerpc/gi4havv/
    • https://bbs.archlinux.org/viewtopic.php?id=234453
• Provide a mixxx-bin that repackages the official Debian packages, these unfortunately only seem to be available for x86_64 and armv7/armhf though (unfortunately there is no multilib on Arch Linux ARM, so we'd need an armv7 chroot to run that...)

[fwcd]

We'll disable the aarch64 build for now with a link to this PR, so we can revisit that in the future.