build: replace build/mk{config,myocamlbuild_config}.sh.

(Patch by Adrien Nader!)

These scripts sed config/Makefile in order to create shell script and
ocaml code that define the same values as the Makefile but in the
corresponding language.
The sed stuff is very difficult to understand, very brittle and
impossible to change.
Remove that altogether and use a Makefile to run commands like
[ echo FOO=$(FOO) ]. There is still some sed involved (to get the list
of variables in the config and to put rewrite the lists into a list of
commands) but much less and all the expressions are put into variables
with meaningful names with lots of comments.

For config.sh, mkconfig.sh generated lines like :
  if [ -z "${FOO}" ]; then FOO=bar; fi
The new script sets the value without checking anything.
I haven't found a reason to do things differently: if there is anything
to override, the right place to do it is after sourcing the shell
script. In any case, I haven't seen any such use.

The mkconfig.sh file also set "WINDOWS=true/false" but the only use is
in the parallel build-system which is implemented in build/ and uses
ocamlbuild as much as possible, and it's going away so it's useless to
set it too.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14167 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02