define USE_ARES nicer if enabled

configure.ac

@@ -970,7 +970,8 @@ AC_HELP_STRING([--disable-ares],[Disable using ares for name lookups]),
          AC_MSG_ERROR([ares doesn't work with ipv6, disable ipv6 to use ares])
        fi
 
-       CPPFLAGS="$CPPFLAGS -DUSE_ARES"
+       AC_DEFINE(USE_ARES, 1, [Define if you want to enable ares support])
+
        ares="yes"
        ;;
   esac ],