Plural vs. singular is always a problem...
But we already have conflicts-sr and conflicts-rr, so counterexamples
makes more sense than counterexample. Besides, -Wcounterexample will
still be accepted as an unambiguous prefix of -Wcounterexamples.
Add -Wcex as a convenient alias.
While at it, use only "counterexample", never "counter example".
* src/complain.h, src/complain.c
(Wcounterexample, warning_counterexample): Rename as...
(Wcounterexamples, warning_counterexamples): these.
(argmatch_warning_docs): Rename -Wcounterexample as -Wcounterexamples.
(argmatch_warning_args): Likewise.
Add support for -Wcex.
Adjust dependencies.