vars/build.vars: wake up disabled packages.
    vars/env.vars: fix ${DEFAULT_BUILD_CPUS} reference.
    vars/{gcc,libfirm,psxstub}.vars: fix ${PKG_MAKEFLAGS_BUILD} reference.
    vars/git.vars:pkg_git_install_post(): process files in destdir.
    subr/pkg_install_libs.subr:pkgp_install_lib_link(): ln(1) w/ target's basename.
    subr/pkg_install_libs.subr:pkg_install_libs(): sanity-check filenames.
    subr/pkg_setup_env.subr:pkg_setup_env(): rm(1) ${PKG_BASE_DIR} during -r.
    etc/build.usage: updated.