merge github.com/ocaml/ocaml/pull/56.diff, from Anil, part 1: fix support for PowerPC/BSD backtraces

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14855 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
merge github.com/ocaml/ocaml/pull/56.diff, from Anil, part 2: configure fixes for BSD

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14856 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Better error message on magic mismatch in Ast_mapper (patch by Peter Zotov).

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14857 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02