This commit moves:
  - config/m.h to byterun/caml/m.h
  - config/s.h to byterun/caml/s.h

Consequently, m.h and s.h now get installed alongside other
OCaml header files.

This commit also updates the .depend files, introducing updates in the
dependencies which are not consequences of this commit itself.

In the interest of keeping HACKING.adoc not-too-long and
general-purpose, advice on modifying specific sub-systems of the
compiler distribution should be moved to the directory of this
sub-system.

This PR also clarifies the relations between the README, INSTALL,
HACKING and CONTRIBUTING documents.

Hint: it is used in the toplevel to highlight errors using something nicer
than a bunch of (slightly-misaligned) '^' on a new line.

NB: $curseslibs ends up in $BYTECCLIBS and we probably link against curses
in the bytecode runtime while we only need it for the toplevel.

Since it does work and is being maintained!

Cygwin is a flavour of Unix. Coupled with the fact that it is built
using the normal Unix configure route, it adds to the confusion of the
Windows ports (especially the fact that the four native ports need a
Cygwin environment to be compiled) including its instructions in the
native port.

Various out-of-date instructions also removed: Cygwin hasn't provided
GCC 4.5.3 for a long time and FlexDLL can be recompiled from sources
without alteration since version 0.32.