slaprunner: Build NumPy/friends & Ruby gems in parallel

Slaprunner tries to leverage multicore, and spawns multiple jobs when compiling software, based on cpu-usage-ratio parameter. But this currently have effect only on Make-based projects (via setting MAKEFLAGS=-j<n>) and does not affect software with different build systems.

Let's also provide support for parallel building for NumPy-based software and Ruby gems out of the box.

@jerome, @cedric.leninivin, @kazuhiko please have a look.

Thanks beforehand,
Kirill