Fix distcheck target

Fix distcheck target

1/ The most annoying part was the error in po/. It turns out that reading
and understanding the po/Makevars file was the key. A simple change of
variable ensures that "make dist" does not try to rebuild lyx.pot.

2/ The way tests in src/ are defined meant that the
dependency-tracking files were not all deleted. This should be OK now.

(cherry picked from commit 9017581ecf26c5e651ee1bccc053285a1b21ca29)