Before, a missing etags caused a build error, and generating the
Lisp preload file and the function registration was first tried
with lingo, and on error done with the shell script. While the
corresponding "command not found" errors were ignored, they were
ugly.

Now, the build checks if etags is there, and if not, ignores that
more gracefully; more or less the same with a pre-existing (or not)
lingo. Much better.