2 Makefile fixes

warnings:

- remove pointless "extern" keyword from some function definitions in
contrib/tsearch2

- use "NULL" not "0" as NULL pointer in contrib/tsearch,
contrib/tsearch2, contrib/pgbench, and contrib/vacuumlo

areas are for the lifetime of the backend and in the interests of not breaking
something that's not broken I left alone.

Note for anyone reading this and wanting it for tsearch-v2-stable (i.e. for 7.3
backend) this patch probably will not apply cleanly to that source. It should
be simple enough to see what's going on and apply the changes by hand if need
be.


--
Nigel J. Andrews