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>

configure.ac

@@ -418,6 +418,7 @@ dnl Detect the language for translations to be installed and check
 dnl that the gettext environment works
 WM_I18N_LANGUAGES
 WM_I18N_XGETTEXT
+WM_I18N_MENUTEXTDOMAIN
 
 dnl 2014/12/29: The option is deprecated, we should keep this message for at
 dnl least 2 years to ensure users see it and update their build scripts
@@ -425,16 +426,6 @@ AC_ARG_WITH([nlsdir], [],
     [AC_MSG_ERROR([option '--with-nlsdir' is deprecated, please use autoconf's standard '--localedir' instead])])
 
 
-menutextdomain=
-AC_ARG_WITH(menu-textdomain, AS_HELP_STRING([--with-menu-textdomain=DOMAIN], [specify gettext domain used for menu translations]),
-	[if test "x$withval" != "xno"; then
-         menutextdomain=$withval
-	 fi])
-AC_SUBST(menutextdomain)
-
-AC_SUBST(NLSDIR)
-
-
 dnl ===========================================
 dnl 		Stuff that uses X
 dnl ===========================================