Works:

```
make -f Makefile.custom CFLAGS="-DFLAG_ONE -DFLAG_TWO" clean target

```

What does not work is

```
FLAGS="-DFLAG_ONE -DFLAG_TWO"
make -f Makefile.custom CFLAGS=$FLAGS clean target

```

Error:
It starts to think that -D is an argument to make and fails.

Tried:
Using escape characters

```
FLAGS="\"-DFLAG_ONE -DFLAG_TWO\""

```

Any help would be appreciated. 

--UPDATE--

This is a workaround but the question still remains open. 

```
CFLAGS="-DFLAG_ONE -DFLAG_TWO"
export CFLAGS
make -f Makefile.custom clean target

```