This change complements the build instructions in the CONTRIBUTING.adoc,
where it is explained to bootstrap the 'stable' engine to
create b2 at the root.
Then, run one of the
two ways to build this be engine.
You can either (a) run b2 b2 at the root,
or (b) run build.sh/bat in src/engine.
Apparently, neither src/engine nor ./src/engine is taken as relative path
by VS Code and leads to task failure:
Executing task: ./build.sh
The terminal process failed to launch: Starting directory (cwd) /src/engine does not exist.
This adds a `b2::system_info` class to obtain available information on
system we are running in. Currently provides CPU counts.
And currently only implemented for macOS.
