If configure.ac does not contain a call to AC_OUTPUT, Automake
does not fail as expected when file(s) specified in calls to
macros AC_LIBSOURCE and/or AC_LIBSOURCES do not exist. See the
attached test script.

Now, while the lack of AC_OUTPUT in configure.ac is almost surely
a user error, IMHO it shouldn't prevent Automake from warning about
other errors (in this case, the non-existence of files specified in
calls to macros AC_LIBSOURCE and AC_LIBSOURCES).

Regards,
  Stefano