Allow the PG_CONFIG environment variable to be used without overridin…

[MarinaPolyakova]

…g it

Otherwise the following command will fail:

$ PG_CONFIG=... make make: pg_config: No such file or directory make: *** No targets. Stop.

Other methods still work:

$ PATH=... make $ make PG_CONFIG=...

[MarinaPolyakova]

@eulerto, what do you think about this?

[eulerto]

@MarinaPolyakova I'm using the same pattern as contrib modules. It is also in the documentation.

Are you planning to submit this change to Postgres?

[ziva777]

Done. See Use "?=" operator for a contrib makefile in documentation

[ziva777]

AFAICS, community is against the idea.