Meson goes to lengths at preventing installation to directories outside of the
main installation prefix. This isn't convenient or desirable in Guix; patch
it out using a patch maintained by Nix.
* gnu/packages/patches/meson-allow-dirs-outside-of-prefix.patch: Add patch.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/build-tools.scm (meson): Use it.
