As the hang seems to be causes by the code messing around in extension
help file directories, of which there are lots in a multi-lingual
build for at least the presenter-screen extension, just build en-US
extension help for now.

Why such a change is needed both here and in
sdext/source/presenter/makefile.mk (see commit
c2a7a6c055cf586b627268d38fd85cde2048db3d in the "extensions" repo) I
don't know, but that's how it seemed in my testing.