do not update po and pot files in the process of the build as this

do not update po and pot files in the process of the build as this
causes timestamp changes for the mo files which therefore can't
be refcounted by dpkg for your M-A: same packages
(Closes: #659333, LP: #924628)

The commit also enables a top-level 'make update-po' and does
all the needed changes to let this work now that update-po might
be called in a freshly checkout tree