install-mans: avoid spurious failure with NetBSD make

install-mans: avoid spurious failure with NetBSD make

* lib/am/mans.am: After the changes introduced by previous commit
v1.11-759-g368f1c4, some manpage installation recipes started to
fail spuriously on NetBSD 5.1 make.  Apparently, the system make
there eats backslashes used to escape whitespace characters in the
relevant recipes.  Revert to slightly slower but safer idioms that
avoid the need of such escaping.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>