When I use the target collect-coverage-data it doesn't work for me. I have changed it on a branch so that it works.
Also the current --enable-coverage sets CPPFLAGS and CXXFLAGS. Instead AM_CPPFLAGS and AM_CXXFLAGS should be used. Also the current --enable-coverage includes all directories, including test directories. This artificially increases the overall coverage numbers, because test code achieves 100% coverage and brings up the average.