ExternalLibraries/Boost doesn't honor parallel build options. This is because it doesn't use 'make' to build, but a tool called 'b2', which isn't using 'make's environment variables, nor is it capable to interact with the jobserver make provides. Manually parsing MAKEFLAGS inside the configure script of the thorn also doesn't work, as this doesn't contain the requested number of jobs, but only the fact that a parallel build was requested and a pointer to the jobserver. Manually interacting with this job server might be possible for gnu make, but is probably messy and highly likely not portable.
However, building a huge package like Boost in parallel is something we have to achieve. On my workstation it reduces the time used for building Boost alone from about 500s to 60s.
The only alternative that is easy enough to implement that I can see right now is to make it possible to pass a number of processes used for building to Cactus (in addition to passing it to make), and Boost using this variable for the parallel build. Simfactory should then also pass this to Cactus. One downside of this would be that if multiple libraries were to be built in parallel you might end up using almost twice as many jobs than specified. On the other hand, this doesn't seem to be the case right now and even if using twice as many might be better than many times too few.