* Enable _GLIBCXX_ASSERTIONS in Debug mode

This allows easier debugging as all lines can be reached. The tsan build uses
"-o1" for performance reasons.
For asan and Debug build we use "-o0", which includes this flag, and for the
release builds we want as much optimisation we can get.

 - Remove Makefile.coverage.in
 - Drop BUILD_COVERAGE_ON_DEBUG for ENABLE_COVERAGE_REPORT (defaults to
    on if included)
 - Add clean-gcda target