It's not enough to copy the ax_code_coverage.m4 file; as explained by
Simon McVittie on gnome-hackers IRC, aclocal will still copy in a newer
version if its #serial number is higher than the existing one.

In order to prevent the macro from being overwritten with a broken
version, we rename the file and also rename the macro.

See #149 for more explanation.

(Unreviewed, pushing to fix build.)