Allowing Local Overrides in Make

I've been setting up a new project at work. I wanted to set up the makefile to allow a programmer to override settings without making changes to the makefiles. Make, unfortunately, does not provide a direct way to do this. I added a shell command to test for the existence of local file, and if found to return it. Make then tests the local file macro and includes the local rules file if defined. Adding the local rules file to svn ignore help to prevent any accidental changes to the makefiles. To illustrate here is the makefile.

## our directories
##
BASE_DIR := $(PWD)
RULES_DIR := rules
DOC_DIR := doc
SRC_DIR := src

## misc definitions
##
KERNEL := $(shell uname -s)
MACHINE := $(shell uname -m)
RULES_FILE := $(KERNEL)_$(MACHINE).mk
LOCAL_RULES := $(shell if [[ -e rules.local ]]; then echo rules.local; fi )

## load rules
##
dir := $(RULES_DIR)
include $(dir)/$(RULES_FILE)

# local overrides
ifdef LOCAL_RULES
include $(LOCAL_RULES)
endif

dir := $(DOC_DIR)
include $(dir)/Rules.mk

dir := $(SRC_DIR)
include $(dir)/Rules.mk