Two small Makefile fixes related to htools

First, fix hs-coverage on non-pristine tree, where the index.html file
already existed, and second, disallow compilation of htools binaries
if configure, for some reason, didn't enable them.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>