cannot be relocatable because of use of @extschema@

relocatable = false

While you hardly need a makefile to install these two files into the correct directory, you could use a Makefile containing this:

EXTENSION = pair DATA = pair--1.0.sql

PG_CONFIG = pg_config PGXS := $(shell $(PG_CONFIG) --pgxs) include $(PGXS)

This makefile relies on PGXS, which is described in Section 36.18. The command make install will install the control and script files into the correct directory as reported by pg_config.

Once the files are installed, use the CREATE EXTENSION command to load the objects into any particular database.