build: Import new ax_code_coverage.m4 macro
ax_code_coverage.m4 was changed incompatibly in recent autoconf-archive versions, and having a new autoconf-archive version installed is enough to get a configure failure. Let's use an internal copy of ax_code_coverage.m4 until the new version is available in most distros.
Signed-off-by: Christophe Fergeau cfergeau@redhat.com