pg_upgrade and pg_config dependency

Àlvaro told me he got a Spanish-language report that pg_upgrade
failed because it required pg_config, and pg_config is only supplied
with the devel packages.

I initially thought that it was a packaging problem, but I later
realized the pg_config is mostly developer settings, and that using
pg_config was not getting any change to libdir by dynamic_library_path
in postgresql.conf, and that I should just merge the pg_upgrade_support
detection code into the existing shared library detection "LOAD" code I
already had.

This avoids the pg_config dependency, works better for libdir, and
reduces the amount of code.

Patch attached.  Should this be backpatched to PG 9.1;  I think so.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +