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>

Makefile.am

@@ -12,13 +12,6 @@ config-paths.h: Makefile
 	@echo "Generating $@"
 	@echo '/* this is a generated file - do not edit */' > $@
 	@echo '' >> $@
-	@echo '/* gettext domain used for menu translations */' >> $@
-	@if test -z "$(menutextdomain)"; then \
-	  echo '/* #undef MENU_TEXTDOMAIN "$(menutextdomain)" */' >> $@; \
-	else \
-	  echo '#define MENU_TEXTDOMAIN "$(menutextdomain)"' >> $@; \
-	fi
-	@echo '' >> $@
 	@echo '/* define where the translations are stored */' >> $@
 	@echo '#define LOCALEDIR "$(localedir)"' >> $@
 	@echo '' >> $@