> But this expectation is not true: if dependencies are not available,
 > Python silently disables the build of certain modules. So this story
> of making the standard library always has all the modules does not really stand.

I think this one is a valid concern. Did run into it myself, because a final 
package was built on another kernel and disabled some semaphore stuff by accident.

None of the extensions above are built as builtins by default, so you can always 
prune these after the build.  What I think is needed is a mode which breaks the 
build when some extensions are not built. Whether this should be the default or 
not, ... but it would give you a chance for a deterministic build.  And the 
build system already tells you at the end of the extension builds that some 
extensions didn't build which were expected to build.