Bug 606720 — Add code coverage support

Add a GNOME_CODE_COVERAGE m4 macro to allow projects to easily add code
coverage support using lcov.

This is heavily based on the code coverage tooling from GLib (LGPLv2.1+),
originally written by Patrick Hulin and modified by Matthias Clasen, Stef
Walter and Simon McVittie since.

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=606720