groups/010.host_deps.group:mpackage_host:${PKG_NO_CLEAN_BUILD_DIR}: don't clean build directory. (via imfatf1sh)
    midipix.env:${DEFAULT_BUILD_VARS}: adds NO_CLEAN_BUILD_DIR.
    subr/pkg_clean.subr: don't clean build directory unless ${PKG_NO_CLEAN_BUILD_DIR} -eq 0.