Fixes #5140: visudo on Debian 6 cannot use strict mode

It seems to be a bit more complex than that. It is a bug in visudo, that was fixed apparently in 1.7.5
http://www.sudo.ws/bugs/show_bug.cgi?id=519
But, some 1.7.2 have this issue, because of backport (for instance, 1.7.2p1-29.el5_10 is ok, but 1.7.2p1-13.el5 is not ...)

the strict checking only checks that alias are defined before being used, BUT since 1.7 it is not necessary anymore.
So the proper solution would be to do the strict check only on version <1.7