Documentation update

Thanks to Alex Susu for some suggestions.
There are still many things left to update.
ocamlbuild: don't traverse useless directories
Configure variables are used only if not defined already
Use DESTDIR in case of FORCE_PERL_PREFIX=1
Fix deprecated logical operators in OCaml 4
Remove conditional variables from Makefile

They serve no purpose since usual variables can
be overwritten by the user as well.
Remove obsolete files
Remove FORCE_PERL_PREFIX

Use the same --prefix for perl scripts as the rest of CIL.
Make ocamlfind respect DESTDIR
Execute ocamlbuild only once per make invocation

This fixes in particular parallel make execution (even
if it does not lead to any speed-up).
Generate cilversion.ml with configure
Remove exeext from cilly perl wrapper
Add .merlin configuration

Merlin provides context-sensitive completion for Ocaml in vim
and emacs.  See https://github.com/def-lkb/merlin for more details.
Fix native and bytecode detection
Fix "received signal" error message in cilly