Makefile does not generate versioned .sql files

When you issue make, it generates following extension initialization scripts
```
pg_cron--1.0--1.1.sql
pg_cron--1.1--1.2.sql
pg_cron--1.0.sql
```

afterwards when you want to to do `create extension pg_cron` you might get following error for older versions of postgres

```
ERROR: could not stat file "/usr/share/postgresql/9.5/extension/pg_cron--1.2.sql": No such file or directory
``
Issue is originally reported by a user on ubuntu 16.04 and PG9.5.

I tested this with PG12 on Mac, it seems okay.