PackageCompiler should use multiple threads

[ufechner7]

Precompilation, but also C++ compilation by default is using multiple threads. Can this also be enabled in PackageCompiler? That would speed up compilation time a lot. :)

[KristofferC]

Building the Julia sysimage multithreaded requires support in Julia itself. Nothing that can really be done from here.

[ufechner7]

Is there a related Julia issue?

[KristofferC]

Parallel compilation of Julia code is worked on quite a bit (especially recently) so I think there is an issue about it. I'll close this one since there isn't anything concrete that can be done here.