Makefile: changing variables based on target before building objects

I would like my Makefile to be able to compile with different flags/compiler depending on the target:

SRC := .
OBJ := .
SOURCES := $(wildcard $(SRC)/*.c)
OBJECTS = $(patsubst $(SRC)/%.c, $(OBJ)/%.o, $(SOURCES))
BINARY := hello_world

all: $(BINARY)

$(BINARY):
  $(CC) $(LDFLAGS) $(OBJECTS) -o $@

$(OBJ)/%.o: $(SRC)/%.c
  $(CC) $(CFLAGS) -I$(SRC) -c $< -o $@

bm: $(OBJECTS)
  $(eval CC := aarch64-none-elf-gcc)
  $(eval CFLAGS += -DBAREMETAL)
  $(eval CFLAGS += -ffreestanding)

linux: $(OBJECTS)
  $(eval CC := aarch64-none-linux-gnu-gcc)

The issue here is that since each target depends on $(OBJECTS) (which I need to compile the objects) the lines that set the CC variable do not get set before evaluating. Thus, when I compile:

> make bm -n
cc hello_world.o -o hello_world

Instead of using the compiler and flags I specified. What's a recommended Makefile structure that would make this work?

Do not try to assign make variables in recipes with eval. Use target-specific variable values if you want to use them in the recipes. Use conditionals on MAKECMDGOALS (as you are apparently using GNU make) or custom make variables if you want to use them in lists of prerequisites.

Example:

SRC     := .
OBJ     := .
SOURCES := $(wildcard $(SRC)/*.c)
OBJECTS := $(patsubst $(SRC)/%.c,$(OBJ)/%.o,$(SOURCES))
BINARY  := hello_world

.PHONY: all bm linux clean

all bm linux: $(BINARY)

bm: CC := aarch64-none-elf-gcc
bm: CFLAGS += -DBAREMETAL -ffreestanding

linux: CC := aarch64-none-linux-gnu-gcc

$(BINARY): $(OBJECTS)
    $(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LDLIBS)

$(OBJ)/%.o: $(SRC)/%.c
    $(CC) $(CFLAGS) -I$(SRC) -c $< -o $@

clean:
    rm -f $(OBJECTS) $(BINARY)

But also consider that if you make bm, followed by make linux, hello_world will not be rebuilt because it is considered as up-to-date. An easy solution is to declare a phony clean target (as above) that deletes all object files and hello_world, and doing a make clean before building hello_world with a different compiler and/or compiler options.

An even cleaner way, as I explained in my answer to your other similar question, is to have 2 different executable files instead of just one, and also different object files. You could, for instance, use different OBJ values for your different settings.