build: don't warn when doxygen isn't found
build: don't warn when doxygen isn't found

Doxygen isn't so important that we need to warn when it is missing. I'd
assume it might even be missing more often than not for most builds.


Former-commit-id: b57db81f7f72019109dc485eb8513d0d86692448
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin