build in paralllel

[rhaas80]

Boost currently builds during CST stage, which is serial in make. We should thus be able to steal make's processes and build in parallel.

GNU make always sets MAKEFLAGS to <otheroptions> -j<NN [--jobserver> so one can parse the number of jobs to use using something like (even if the user gave --jobs 42 or --jobs:

JOBS=$(echo "$MAKEFLAGS" | perl -n -e 'm/.*(-j[0-9]+)/ and print $1')

then pass $JOBS to b2:

b2 $JOBS