cbc8e0 etc/build.usage: adds -P [jobs].

Authored and Committed by Lucio Andrés Illanes Albornoz (arab, vxp) 6 years ago
    etc/build.usage: adds -P [jobs].
    subr/build_args.subr: adds `optarg' arg type & ARG_PARALLEL:optarg:-P:auto.
    subr/build_init.subr: default ${ARG_PARALLEL} ([jobs]) to ${DEFAULT_BUILD_CPUS}.
    subr/ex_pkg_dispatch.subr: honour ${ARG_PARALLEL}, defaulting to no parallelisation.
    
        
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