MC build on HPUX workaround

[Dan Newcombe <Newcombe mordor clayton edu>]

On mc (from cvs real recent) on HPUX it fails building in
gnome-file-properties-dialog.c on the lines with rindex.  I sent an e-mail
on this a few days ago.

The problem is that rindex is not defined without including strings.h,
which that .c does not.  Once I put that in there, it worked just fine.

I noticed that mem.h had some code to include string.h, and if rindex was
not defined, then to define it as strrchr.  However, this is not included
either.

Just an FYI.