Refactor Makefile rules for building the extensions under pgxn/ (#12305)

pgxn/Makefile

@@ -0,0 +1,28 @@
+# This makefile assumes that 'pg_config' is in the path, or is passed in the
+# PG_CONFIG variable.
+#
+# This is used in two different ways:
+#
+# 1. The main makefile calls this, when you invoke the `make neon-pg-ext-%`
+#    target. It passes PG_CONFIG pointing to pg_install/%/bin/pg_config.
+#    This is a VPATH build; the current directory is build/pgxn-%, and
+#    the path to the Makefile is passed with the -f argument.
+#
+# 2. compute-node.Dockerfile invokes this to build the compute extensions
+#    for the specific Postgres version. It relies on pg_config already
+#    being in $(PATH).
+
+srcdir = $(dir $(firstword $(MAKEFILE_LIST)))
+
+PG_CONFIG = pg_config
+
+subdirs = neon neon_rmgr neon_walredo neon_utils neon_test_utils
+
+.PHONY: install install-compute install-storage $(subdirs)
+install: $(subdirs)
+install-compute: neon neon_utils neon_test_utils neon_rmgr
+install-storage: neon_rmgr neon_walredo
+
+$(subdirs): %:
+	mkdir -p $*
+	$(MAKE) PG_CONFIG=$(PG_CONFIG) -C $* -f $(abspath $(srcdir)/$@/Makefile) install