postgresql detection code improvement.

[hacker]

It's better to detect postgresql using pg_config as besides "doing the right way" it would also allow to select postgresql installation to be used.

Not only "I have files to attach to this ticket", but you should also expect more postgresql enhancement, since the module seems to lack userbase somewhat, at least none of those trying to combine ssl and postgresql authreg ;-)

[hacker]

patch against current svn with a wordy comment.

[smoku]

(In [325]) Applied pg_config using by configure patch by Michael Krelin. Closes #120