coverage: Use absolute path when removing paths with lcov

There is a bug in some versions of lcov that causes it to fail writing
to files via relative paths after it issued warnings (e.g. due to
negative counts in the tracefile).

Makefile.am

@@ -59,7 +59,7 @@ cov-report:
 		lcov -c -o $(top_builddir)/coverage/coverage.info -d $(top_builddir) \
 			 --rc lcov_branch_coverage=1
 		lcov -r $(top_builddir)/coverage/coverage.info '*/tests/*' '*/suites/*' '/usr*' \
-			 -o $(top_builddir)/coverage/coverage.cleaned.info \
+			 -o $(abs_top_builddir)/coverage/coverage.cleaned.info \
 			 --rc lcov_branch_coverage=1
 		genhtml --num-spaces 4 --legend --branch-coverage --ignore-errors source \
 				-t "$(PACKAGE_STRING)" \