Use the default pg_config location

Makefile

@@ -17,7 +17,6 @@ TAP_TESTS = 1
 PG_LIBS_INTERNAL += $(libpq_pgport)
 
 ifdef USE_PGXS
-PG_CONFIG ?= pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
 include $(PGXS)
 else
@@ -71,6 +70,8 @@ install-pg-probackup:
 	$(MAKE) -C $(pg_probackup_dir) install USE_PGXS=$(USE_PGXS) PG_CONFIG=$(PG_CONFIG) top_srcdir=$(top_srcdir)
 
 PYTEST_PROCESSES ?= $(shell nproc)
+# Python tests read pg_config only from PG_CONFIG
+test-python: PG_CONFIG ?= pg_config
 test-python:
 	cd $(pg_probackup_dir); \
 	env="PG_PROBACKUP_PTRACK=ON PG_CONFIG=$(PG_CONFIG)"; \

README.md

@@ -220,6 +220,7 @@ You can control how tests are executed by using the following environment (or `m
 
 | Variable  | Possible values          | Required | Default value  | Description |
 | -         | -                        | -        | -              | -           |
+| PG_CONFIG | File path | No | pg_config for `test-python`, not set for other targets | The path to the `pg_config` binary. Can be set to specify a custom location of `pg_config`. |
 | TESTS     | A Pytest filter expression | No | Not set (run all Python tests) | A filter to include only selected tests into the run. See the Pytest `-k` option for more information. This variable is only applicable to `test-python` for the tests located in [tests](https://github.com/postgrespro/pg_probackup/tree/master/tests). |
 | TEST_MODE | normal, legacy, paranoia | No       | normal         | The "legacy" mode runs tests in an environment similar to a 32-bit Windows system. This mode is only applicable to `test-tap`. The "paranoia" mode compares the checksums of each block of the database catalog (PGDATA) contents before making a backuo and after the restoration. This mode is only applicable to `test-python`.|