Fix make neon-pgindent (#12535)

## Problem

`make neon-pgindent` doesn't work:
- there's no `$(BUILD_DIR)/neon-v17` dir
- `make -C ...` along with relative `BUILD_DIR` resolves to a path that
doesn't exist

## Summary of changes
- Fix path for to neon extension for `make neon-pgindent`
- Make `BUILD_DIR` absolute
- Remove trailing slash from `POSTGRES_INSTALL_DIR` to avoid duplicated
slashed in commands (doesn't break anything, it make it look nicer)

Makefile

@@ -2,7 +2,7 @@ ROOT_PROJECT_DIR := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
 
 # Where to install Postgres, default is ./pg_install, maybe useful for package
 # managers.
-POSTGRES_INSTALL_DIR ?= $(ROOT_PROJECT_DIR)/pg_install/
+POSTGRES_INSTALL_DIR ?= $(ROOT_PROJECT_DIR)/pg_install
 
 # Supported PostgreSQL versions
 POSTGRES_VERSIONS = v17 v16 v15 v14
@@ -14,7 +14,7 @@ POSTGRES_VERSIONS = v17 v16 v15 v14
 # it is derived from BUILD_TYPE.
 
 # All intermediate build artifacts are stored here.
-BUILD_DIR := build
+BUILD_DIR := $(ROOT_PROJECT_DIR)/build
 
 ICU_PREFIX_DIR := /usr/local/icu
 
@@ -212,7 +212,7 @@ neon-pgindent: postgres-v17-pg-bsd-indent neon-pg-ext-v17
 		FIND_TYPEDEF=$(ROOT_PROJECT_DIR)/vendor/postgres-v17/src/tools/find_typedef \
 		INDENT=$(BUILD_DIR)/v17/src/tools/pg_bsd_indent/pg_bsd_indent \
 		PGINDENT_SCRIPT=$(ROOT_PROJECT_DIR)/vendor/postgres-v17/src/tools/pgindent/pgindent \
-		-C $(BUILD_DIR)/neon-v17 \
+		-C $(BUILD_DIR)/pgxn-v17/neon \
 		-f $(ROOT_PROJECT_DIR)/pgxn/neon/Makefile pgindent