How to clone submodule during build?

[weskoerber]

@castholm

The build system is logically divided up into a “configure” phase which calls your build function in order to construct the graph of steps and learn which top-level steps are available, and a “make” phase which invokes the steps. The “configure” phase is not intended to know about which steps the user selected.

You need the submodules to be available before the make phase, so you must initialize them during the configure phase. addSystemCommand creates a step that executes during the make phase of the build process, not during the configure phase. Fortunately, there is b.run which runs during the configure phase.