Lack of recognition of --enable-mule with configure.ac, autoconf 2.62

[Stephen J. Turnbull]

Aidan Kehoe writes:

 > It’s a (minor) problem with autoconf versions greater than or equal to 2.62. 
 > See http://mid.gmane.org/200812281257.53590.scop@xemacs.org , a message I
 > replied to, if it ever makes it to gmane. It shouldn’t interfere with
 > building XEmacs, but it is something that ideally should be resolved, if
 > only to avoid this sort of bug report.

Yeah.  I hate the AutoFools.