This uses CODE_COVERAGE_LIBS instead of CODE_COVERAGE_LDFLAGS; the two
are equivalent but CODE_COVERAGE_LDFLAGS is from an older version of the
code coverage macro and discouraged.

Also uses CODE_COVERAGE_CXXFLAGS for C++ compilation.

In some cases the flags weren't being passed to all compilations because
AM_*FLAGS is not used by default if there are per-target flags.