config: replace deprecated AC_HELP_STRING with AS_HELP_STRING

Signed-off-by: Gaetan Nadon <memsize@videotron.ca>

configure.ac

@@ -47,7 +47,7 @@ AC_PROG_LIBTOOL
 AH_TOP([#include "xorg-server.h"])
 
 AC_ARG_WITH(xorg-module-dir,
-            AC_HELP_STRING([--with-xorg-module-dir=DIR],
+            AS_HELP_STRING([--with-xorg-module-dir=DIR],
                            [Default xorg module directory [[default=$libdir/xorg/modules]]]),
             [moduledir="$withval"],
             [moduledir="$libdir/xorg/modules"])