tools/configure.ac: Replace macro AC_HELP_STRING

... with AS_HELP_STRING as the former is obsolete according
to GNU autoconf 2.67 documentation.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>

tools/configure.ac

@@ -215,7 +215,7 @@ AC_SUBST(qemu_xen_path)
 AC_SUBST(qemu_xen_systemd)
 
 AC_ARG_WITH([stubdom-qmp-proxy],
-    AC_HELP_STRING([--stubdom-qmp-proxy@<:@=PATH@:>@],
+    AS_HELP_STRING([--stubdom-qmp-proxy@<:@=PATH@:>@],
         [Use supplied binary PATH as a QMP proxy into stubdomain]),[
     stubdom_qmp_proxy="$withval"
 ],[