tail_build run mostly by itself and wrap a dozen of module,
using just MAXPROCESS for the parallelism force to limit NB_CPUS
in order to avoid a NB_CPUS x MAXPROCESS scenario.
This mitigate this problem, until we don;t need MAXPROCESS anymore
and NB_CPUS becomes the only driving force.
gbuild allow for multiple 'module' to be build as one Makefile.
This help with parallelism.
This module build all the 'tail' end module. these that are not
dependencies or other dmake-based module, and that pseudo module
is declared as a dependency of postprocess instead of the individual
modules it 'group'
