subr/pkg_build.subr: pass either ${PKG_MAKEFLAGS_VERBOSITY} or nothing if set to `none'.
groups/010.host_deps.group:ruby_host:${PKG_MAKEFLAGS_VERBOSITY}: build w/ none.
groups/221.native_packages_dev.group:ruby:${PKG_MAKEFLAGS_VERBOSITY}: build w/ none.
etc/README.md, midipix.env: adds ${PKG_MAKEFLAGS_VERBOSITY}, defaulting to `V=99'.