build.sh, subr/ex_pkg_dispatch.subr: splits build logging and job control logic.
    subr/ex_pkg_dispatch.subr: dispatch background jobs w/ redirected std{err,out} & FIFO.
    subr/ex_pkg_state.subr:ex_pkg_state_init(): trap(1) signals to abort build.
    vars/{gcc,musl}.vars: don't exit(1) from within package subroutine.