Make sure that pg_config is in your PATH and then run make install (possibly
as superuser depending on how/where PostgreSQL is installed).

For example:

$ PATH=/where/is/postgres/bin:$PATH make install

Then in psql:

postgres=# create extension calendars;
CREATE EXTENSION
postgres=# select to_char(current_date, 'YYYY-MM-DD', 'Ethiopian');
                    to_char                    
-----------------------------------------------
 <put Ethiopian date for day number 7160 here>
(1 row)

postgres=# select to_date('....', 'YYYY-MM-DD', 'Ethiopian');
  to_date   
------------
 2000-01-01
(1 row)

TODO:

* Real implementations of the functions in ethiopian.c
* Perhaps some other calendars from https://en.wikipedia.org/wiki/List_of_calendars