build: do not fail due to an existing read-only hello.1

* Makefile.am (hello.1): In Makefile rules, never redirect directly
to the target.  Instead, redirect to a temporary, $@-t, and once
that succeeds, rename to target using "mv -f".
Also, since this is a generated file, explicitly make it read-only,
so one is less likely to think it is an editable primary source.
Reported by Assaf Gordon.