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Makefile for specific variables as prerequisites

Related: Target Variables as Prerequisites in Makefile

I am trying to create a Makefile that uses a target variable to specify the output directory for object files and the final executable. The idea is to support two separate binary versions, a release version and a debug version with additional debugging information.

My problem is that make makes a clean build every time, even if I haven't changed anything. I am sure that “make” evaluates the prerequisites of the target “corewars” before declaring the variable in the prerequisites for the “debug” or “release” target.

The Makefile is presented below.

CXX=g++
LD=g++
LDFLAGS=
CXXFLAGS=-Iinclude -Wall -Wextra
OBJECTS=main.o Machine.o Core.o ProcessQueue.o Instruction.o
OUTPUT_DIR:=Test/

.PHONY: default
.PHONY: all
.PHONY: release
default: release
all: release
release: OUTPUT_DIR:=Release/
release: corewars

.PHONY: debug
debug: CXXFLAGS+=-DDEBUG -g
debug: OUTPUT_DIR:=Debug/
debug: corewars

corewars: $(OUTPUT_DIR) $(addprefix $(OUTPUT_DIR),$(OBJECTS))
    $(LD) -o $(addprefix $(OUTPUT_DIR),corewars) $(addprefix $(OUTPUT_DIR),$(OBJECTS))

Release:
    mkdir -p $@
Debug:
    mkdir -p $@

%.o: %.cpp include/%.h
    $(CXX) -c $(CXXFLAGS) $< -o $(OUTPUT_DIR)$@


.PHONY: clean
clean:
    $(RM) -r Release
    $(RM) -r Debug
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1

, . - , $@, $(OUTPUT_DIR)$@.

.

, $(OUTPUT_DIR) , , OUTPUT_DIR:=Test/ .

, Makefile vpath. ( OUTPUT_DIR ..), Make .

ifndef OUTPUT_DIR

.PHONY: default all release debug

default all: release

release: export OUTPUT_DIR := Release/
debug:   export OUTPUT_DIR := Debug/
debug:   export EXTRA_CXXFLAGS := -DDEBUG -g

release debug:
    @$(MAKE)

else

# ...
CXXFLAGS := -Iinclude -Wall -Wextra $(EXTRA_CXXFLAGS)

PROGRAM := $(OUTPUT_DIR)corewars
OBJECTS := $(addprefix $(OUTPUT_DIR), \
    main.o Machine.o Core.o ProcessQueue.o Instruction.o)

# Default target.
$(PROGRAM): $(OBJECTS) | $(OUTPUT_DIR)
    $(LD) -o $@ $<

$(OUTPUT_DIR)%.o: %.cpp | $(OUTPUT_DIR)
    $(CXX) -c $(CXXFLAGS) $< -o $@

$(OUTPUT_DIR):
    mkdir -p $@

endif # OUTPUT_DIR
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