ax_code_coverage.m4 API breakage plus errors

I am a maintainer of c-ares and recently hit a few issues regarding 
ax_code_coverage.m4.

First, when running  autoreconf -fi  on a new system, it replaces 
ax_code_coverage.m4 in our archive with a newer one, which is normally 
fine.  However the API has changed with the removal of 
@CODE_COVERAGE_RULES@.  This broke building of course for a lot of 
package managers on newer systems.  If possible you might want to try 
restoring @CODE_COVERAGE_RULES@ with some compatibility layer for other 
users of this, its obviously no longer relevant to us since we've since 
adapted.

Next, after fixing the integration issue (and thus necessitating pulling 
the latest version into our repository along with a slew of new m4 
dependencies), while testing on *older* systems, it uncovered an issue 
in the latest ax_code_coverage.m4 where AC_MSG_ERROR() was being quoted 
and older versions of autoconf/automake couldn't support that.  We 
applied a patch locally for that in our repository:

https://github.com/c-ares/c-ares/commit/3883d99b05a845e4b40fc25c43ca83db70f4f20f#diff-fe38622a6c742be1bb93e6e65a0728f659114b3dc8d88172cb8844fa963418f9

Thanks.
-Brad