Compile the emacs mode if emacs is available
Install the emacs mode at the right place
Emacs mode: Provide the good module name (why3-mode => why3)
Fix the clean_old_install Makefile rule
Add an uninstall rule
Sanitize the Makefile
Begin to abstract for inserting a C backend
Create the module Mlw_c for the C backend and fill Mlw_c.extract_filename