Implement auto concurrency

[drok]

Autotools already supports build and test concurrency (make -j<numjobs>), which can be enabled by adding export MAKEFILE=-j$(numproc) in the host's environment, or other places, but this is problematic:

• link steps often require much more memory than compile
• valgrind tests require much more memory

This means that compilation can run with higher parallelism than linking or valgrind testing. Typically in an environment that uses valgrind, or builds large binaries, some manual tweaking is done to do more work in less time.

The Autotools Integration is the ideal place to track statistical resource usage, and apply reasonable jobserver feedback to the build, and do this without the tweaking.

This feature makes sense wherever time is money.