configure: rewrote the support for option '--with-menu-textdomain'
The idea is to include consistency check to warn the user if he is not
using it properly, instead of silently misbehaving;
Updated code style to use Autoconf macros for consistency.
Took opportunity to remove the hacky setting inside 'config-paths.h' where
it is a bit out of place, in favour of a standard DEFINE in the 'config.h'.
Include in the i18n documentation the explanation on how Window Maker
translates the menus, both when the 'menu-textdomain' option is used and
when it is not.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>