Use AX_CODE_COVERAGE in autoconf-archive instead of gnome-common
As per https://mail.gnome.org/archives/commits-list/2014-August/msg08906.html GNOME_CODE_COVERAGE
is deprecated, and AX_CODE_COVERAGE
in autoconf-archive should be used.
They're basically identical so ports are trivial, for example see https://mail.gnome.org/archives/commits-list/2014-August/msg08906.html.