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.