pgbench: undefined reference to strndup()

A recent pgbench commit causes build failures due to an undefined
reference to strndup(), at least on FreeBSD and UnixWare:

http://pgbuildfarm.org/cgi-bin/show_log.pl?nm=echidna&dt=2005-09-29%2014:30:01
http://pgbuildfarm.org/cgi-bin/show_log.pl?nm=firefly&dt=2005-09-29%2014:27:00

I don't see strndup() in the Open Group Base Specifications, so I
expect other platforms' builds might fail as well.

-- 
Michael Fuhr