extension_control_path

The new GUC extension_control_path specifies a path to look for
extension control files.  The default value is $system, which looks in
the compiled-in location, as before.

The path search uses the same code and works in the same way as
dynamic_library_path.

Some use cases of this are: (1) testing extensions during package
builds, (2) installing extensions outside security-restricted
containers like Python.app (on macOS), (3) adding extensions to
PostgreSQL running in a Kubernetes environment using operators such as
CloudNativePG without having to rebuild the base image for each new
extension.

There is also a tweak in Makefile.global so that it is possible to
install extensions using PGXS into an different directory than the
default, using 'make install prefix=/else/where'.  This previously
only worked when specifying the subdirectories, like 'make install
datadir=/else/where/share pkglibdir=/else/where/lib', for purely
implementation reasons.  (Of course, without the path feature,
installing elsewhere was rarely useful.)

Author: Peter Eisentraut <peter@eisentraut.org>
Co-authored-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: David E. Wheeler <david@justatheory.com>
Reviewed-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>
Reviewed-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>
Reviewed-by: Niccolò Fei <niccolo.fei@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/E7C7BFFB-8857-48D4-A71F-88B359FADCFD@justatheory.com

src/backend/utils/misc/postgresql.conf.sample

@@ -804,6 +804,7 @@ autovacuum_worker_slots = 16	# autovacuum worker slots to allocate
 # - Other Defaults -
 
 #dynamic_library_path = '$libdir'
+#extension_control_path = '$system'
 #gin_fuzzy_search_limit = 0