Makefile: use pg_config path from environment variable instead of hardcoded binary

[palash25]

The use of pg_config binary was inconsistent in the Makefile, I realized this while building pg_vectorize locally, it installs pgmq in its make setup step and supplies the pg_config path as an env var which was getting ignored here due to the use of hardcoded path, which would fail on machines that don't have postgres installed and pg_vectorize setup would fail with this error:

/bin/sh: 1: pg_config: not found
/bin/sh: 1: test: -ge: unexpected operator

[v0idpwn]

Thank you!