Generation of unit test coverage reports used to be quite complicated
and required a lot of different settings, including a custom CMake
build type. This patch updates the coverage CMake module to only
require -DWITH_COVERAGE=ON to be set on a normal Debug build in order
to create a coverage target.
This patch also moves away from lcov in favor of gcovr, since lcov appears
to be broken in GCC 8. However, the routines for generating lcov reports
still exist, so provided lcov receives updates and there is sufficient
reason to switch back, it is easy to do so.
