Use "?=" operator for a contrib makefile in documentation

Hi!

Please give notice to this discussion [0].

I think that the Makefile should be written so that variable values,
specifically, PG_CONFIG, can be given to it from the environment rather
than the make command line. As a result, using the "?=" operator rather
than "=" to set a default value to the PG_CONFIG variable appears more
acceptable.

I'm not sure if this operator has to be changed in the main Postgres
source tree; after all, they're typically built with the kernel.

[0] https://github.com/eulerto/wal2json/pull/290
[1]
https://www.postgresql.org/docs/current/extend-extensions.html#EXTEND-EXTENSIONS-EXAMPLE

-- 
Best regards,
Maxim Orlov.