comment after trailing backslash inconsistent warning

[Karl Berry]

automake 1.10.2 complains about "comment after trailing backslash"
sometimes, but not every time.  In the appended Makefile.am, I get 
Makefile.am:23: comment following trailing backslash
which is true enough, but there are also comments after trailing
backslashes at line 124 and line 108.  And if I delete the one at line
23, it still does not complain about the others.

Perhaps it's some subtle portability issue I do not know, but I thought
I'd report it.

Also, I don't want that message at all.  Can/Should it be turned off?
It is useful to have lists with a trailing \ on each line for the usual
separator vs. terminator reasons.  In my case, the whole tree requires a
pretty new GNU make anyway, so I assume whatever portability issue is
being warned about does not apply.  At least I've never seen any problem
with the generated files.

Thanks,
Karl