remove unnecessary Obj.magic (Grégoire Henry)

From: Gabriel Scherer <gabriel.scherer@gmail.com>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15664 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Since r10c, Android NDK includes sys/ucontext.h.

From: Peter Zotov <whitequark@whitequark.org>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15665 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Allow Android builds with systhreads support.

From: Peter Zotov <whitequark@whitequark.org>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15666 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Update manpage

From: Hugo Heuzard <hugo.heuzard@gmail.com>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15667 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02