Makefile: gather commands related to the installation of manual pages
Makefile: gather commands related to the installation of manual pages

This commit also makes sure the man pages get installed only on Unix
systems. This is useless at the moment but will become useful once the
install rule will be shared.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml