gcov code coverage test support
We're using "make check" tests to verify that new changes aren't breaking smth. Those tests are not perfect but we don't have any measure as to "how much exactly" it's far from perfect. Because of that we also don't know if we're getting better or worse as time progress.
The standard way of doing this is by using gcov tool to produce code coverage analysis and lcov to generate nice report. This will require changes to ./configure step - see https://www.gnu.org/software/autoconf-archive/ax_code_coverage.html and corresponding additions to Makefiles.
Ideally this should be integrated with CI to see long-term trends similar to the way amount of warnings is displayed.