[krbdev.mit.edu #3866] gld --as-needed not portable enough

[Ken Raeburn via RT]

I had added the GNU linker option --as-needed to shlib.conf a while  
back for GNU/Linux targets.  This is just to note that there are  
configurations still in use out there with GNU linkers old enough  
that --as-needed may not work.  Since it's a minor optimization, it  
should probably be pulled until we can add a configure-time test.

Ken