Add strong-build dependency

Without this there is an implicit depenency on strongloop, which would
be a recursive dependency.

This allows strong-arc to be installed and used directly instead of
requiring it to be run as 'slc arc', which will be the case if it
it installed globally and run with 'strong-arc'.
pm: use process.execPath

Make strong-arc more resilient to node not being on the PATH, which is
possible if strong-arc is invoked by full path.