Commit d536c57e87 inadvertedly broke build and
installation of all non-autogenerated manual pages (in particular, all the ones
documenting the stable CLI), by moving the definition of the man-pages variable
in doc/manual/local.mk after its usage in mk/lib.mk. Move including the former
earlier so that the correct order is restored.
(cherry picked from commit 8f3253c6f4)