BUG #7548: pg_upgrade still requires pg_config

The following bug has been logged on the website:

Bug reference:      7548
Logged by:          Reinhard Max
Email address:      max@suse.de
PostgreSQL version: 9.1.5
Operating system:   openSUSE
Description:        

It looks like the attempt to remove pg_upgrade's dependency on pg_config
(http://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=2815574) was
incomplete, since contrib/pg_upgrade/exec.c still contains the line

 validate_exec(cluster->bindir, "pg_config");

which makes pg_upgrade refuse to run when pg_config can't be found.