is.na(valid_date) too often true on SGI MIPS (PR#6814)

It would be helpful to sprinkle a few reminders of the existence of
Olson's data in the documentation.

From a posting to comp.sys.sgi.admin I learned that SGI does use Olsen's

database in the "feature" stream (e.g., 6.5.Nf, N>2).  The machines I use
have 6.5.19m and 6.5.21m, but two systems with 6.5.22m have Olsen's
database (see: "man zdump").

$ /sbin/uname -aR
IRIX64 [...] 6.5 6.5.22m 10070055 IP30
$ showfiles eoe.sw.base | grep TZ
f 11649 20003 eoe.sw.base             usr/lib/locale/TZ/africa
f 45578 26997 eoe.sw.base             usr/lib/locale/TZ/asia
f 11309 27954 eoe.sw.base             usr/lib/locale/TZ/australasia
f 56271  2985 eoe.sw.base             usr/lib/locale/TZ/backward
f 37003  1687 eoe.sw.base             usr/lib/locale/TZ/etcetera
f 21755 88582 eoe.sw.base             usr/lib/locale/TZ/europe
f 38515   311 eoe.sw.base             usr/lib/locale/TZ/factory
f 25667 35826 eoe.sw.base             usr/lib/locale/TZ/northamerica
f  4425  1107 eoe.sw.base             usr/lib/locale/TZ/pacificnew
f 15976 19159 eoe.sw.base             usr/lib/locale/TZ/solar87
f 39829 19177 eoe.sw.base             usr/lib/locale/TZ/solar88
f 32211 19453 eoe.sw.base             usr/lib/locale/TZ/solar89
f 29073 15106 eoe.sw.base             usr/lib/locale/TZ/southamerica

I can't seem to find this mentioned in the announcments, so I'm not
sure if these two systems were switched from "f" to "m" at some
point or if SGI put a little gift into 6.5.22m.  I suppose
configure needs to check for a decent tz database, but maybe it
is enough to add a little note to the Irix section of the install.

--
George N. White III  <aa056@chebucto.ns.ca>
  Head of St. Margarets Bay, Nova Scotia, Canada