Update CFLAGS or LDFLAGS in makefile depending on current OS

Question

I'm trying to create a makefile that will works on both OSX and Linux.

My problem is I need to change cflags and ldflags depending on OS the makefile is executed but I can't make it work. That's my makefile :

OS:=$(shell uname)
DST=hello
SRC=$(wildcard *.cpp)
OBJ=$(SRC:.cpp=.o)
CFLAGS=

all: clean DetectOS $(DST)

DetectOS:
ifeq ($(OS),Darwin)
    @echo OS : $(OS)
    CC=g++
    LDFLAGS="-lm -framework OpenCL"
    CFLAGS+=-O3
endif

ifeq ($(OS),Linux)
    #Coming soon...
endif

$(DST): $(OBJ)
    $(CC) -o $@ $^ $(LDFLAGS)

%.o: %.cpp
    $(CC) -o $@ -c $< $(CFLAGS)

clean:
    rm -rf *.o $(DST)

But when I run this code, neither cflags, ldflags or CC are updated in the ifeq conditional block. I get the following result :

$ make
rm -rf *.o hello
OS : Darwin
CC=g++
LDFLAGS="-lm -framework OpenCL"
CFLAGS+=-O3
cc -o opencl.o -c opencl.cpp 
cc -o hello opencl.o 
Undefined symbols for architecture x86_64:....

As you can see, the OS is detected because we went in the ifeq conditional block, but CC isn't updated and keep a non-initialized value of cc. Finally the linker process fails because OpenCL is not referenced in ldflags.

An other little point, if I don't put quotes in LDFLAGS="-lm -framework OpenCL" I get the error :

LDFLAGS=-lm -framework OpenCL
/bin/sh: -framework: command not found
make: *** [DetectOS] Error 127

And based on multiple exemples (here on stackoverflow) I should do it without quotes.

I'm currently on Mac OS X Yosemite.

Answer 1

I think the ifeq block should not go into the make target, but just to the front of the makefile (before all:).

I.e. like this:

OS:=$(shell uname)
DST=hello
SRC=$(wildcard *.cpp)
OBJ=$(SRC:.cpp=.o)
CFLAGS=

ifeq ($(OS),Darwin)
    $(info OS is $(OS))
    CC=g++
    LDFLAGS=-lm -framework OpenCL
    CFLAGS+=-O3
endif

ifeq ($(OS),Linux)
    #Coming soon...
endif

all: clean $(DST)

...

(no "DetectOS" target, using info instead of echo)