autoconf: Use AS_HELP_STRING if available, fixes warnings for any recent

version of autoconf.

acinclude.m4

@@ -4,7 +4,7 @@ dnl Autoconf 2.50 can not handle substr correctly.  It does have
 dnl AC_HELP_STRING, so let's try to call it if we can.
 dnl Note: this define must be on one line so that it can be properly returned
 dnl as the help string.
-AC_DEFUN([APACHE_HELP_STRING],[ifelse(regexp(AC_ACVERSION, 2\.1), -1, AC_HELP_STRING($1,$2),[  ]$1 substr([                       ],len($1))$2)])dnl
+AC_DEFUN([APACHE_HELP_STRING],[ifdef([AS_HELP_STRING], AS_HELP_STRING($1, $2),[ifelse(regexp(AC_ACVERSION, 2\.1), -1, AC_HELP_STRING($1,$2),[  ]$1 substr([                       ],len($1))$2)])])dnl
 
 dnl APACHE_SUBST(VARIABLE)
 dnl Makes VARIABLE available in generated files