Only setting PG_CONFIG if it hasn't already been set.

[tonyarkles]

My machine has two different copies of libpg installed on it. Specifying PG_CONFIG
should encourage the build to use the value of that for calling pg_config.

[markokr]

Variables from environment or command line already override variables in Makefile.

?= is only useful for checking if variable is defined earlier in same makefile.

So this patch is unnecessary.

Or does it actually change anything?

[tonyarkles]

Hmmmm. That's really interesting. I'm going to have to have a look at it again. From what I remember, it wouldn't accept a different PG_CONFIG without that change (it would just use the system default).

[markokr]

Seems unnecessary. Closed.