--with-theme is without trailing s

Signed-off-by: Lionel Elie Mamane <lionel@mamane.lu>

configure.in

@@ -8039,7 +8039,7 @@ WITH_THEMES=""
 for theme in $with_theme; do
     case $theme in
         default|crystal|hicontrast|oxygen|tango) : ;;
-        *) AC_MSG_ERROR([Unknown value for --with-themes: $theme]) ;;
+        *) AC_MSG_ERROR([Unknown value for --with-theme: $theme]) ;;
     esac
     WITH_THEMES="$WITH_THEMES $theme"
     SCPDEFS="$SCPDEFS -DTHEME_`echo $theme|tr '[[:lower:]]' '[[:upper:]]'`"