Bring over the build.py script and its associated scripts from the
scripts subdirectory of the jami-project.git as of commit
14a328b88007bff5ea21650fb00f6d4ead14cf02, with small changes to
simplify and drop unneeded parts and adapt to their new locations.

Change-Id: I500702854e45392a931ff7505e191b966015cd16