configure.ac: only enable code coverage when available
configure.ac: only enable code coverage when available

When building with an older gnome-common which does not yet provide code
coverage support, disable it instead of breaking the configure script.

https://bugzilla.gnome.org/show_bug.cgi?id=693328

sfeltman/pygobject,sfeltman/pygobject,sfeltman/pygobject,pexip/pygobject,davibe/pygobject,GNOME/pygobject,davibe/pygobject,davibe/pygobject,pexip/pygobject,GNOME/pygobject,GNOME/pygobject,pexip/pygobject,davibe/pygobject