[xslt] thread-safe function for libexslt

["William M. Brack" <wbrack mmm com hk>]

There is an outstanding bug report
(http://bugzilla.gnome.org/show_bug.cgi?id=129983) requesting that
the call within libexslt to the function "localtime" be changed to a
call to "localtime_r" when libxslt is generated with threading
enabled.  That is certainly quite alright on my system, but I'm not
certain whether it might have any "bad" effect on some other OS's. 
Does anyone have any good reason why I should not go ahead and
commit this change?

Bill