Makefile: improve the fix for .DELETE_ON_ERROR

I've investigated more and it seems that .DELETE_ON_ERROR not working
is a bug/regression in some versions of make; testing with make 3.82
from Debian experimental makes this special target work as
expected. Also, make 3.81 works on trivial Makefiles, but not on our
complex one.

In the meantime, let's improve the fix for the building of man pages:
make the removal implicit, instead of explicit, via the use of 'trap'
on EXIT. I just hope this behaves consistently across all bash
versions :)

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