Add pieces of Sconscript file for building GI data
Compile addons with full warnings and error reporting with scons

That's how the Makefile build does it. Note that we cannot compile the library
itself the same way because the boilerplate code has too many false positives.
GI files need the namespace version in their names

Otherwise they're not recognized as GI files.
Leave (majority of) building to scons
Make explicit build dependency between boiler code and the library

We need the boilerplate code before we build the library. And since it is not a
common thing to do, scons fails to detect that dependency automatically.