build: allow installing man(1) even with --disable-doc

Currently these man pages are installed only if --enable-doc
is provided, while these are not actually docs, do not require any
special dependency, nor consume large space.

This adds --enable-manpages to enable/disable manpages installation, and
install the man(1) regardless of --disable-doc.

Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>
Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>