removing compile directive

[Mohan Lal Jangir]

I can use operator "+=" to provide additional compile directives, like

CFLAGS += -D_EXTRA_

Is there a way to remove compile directive? Something like this

CFLAGS -= -D_REMOVE_THIS_

I have a source tree being compiled with -O2 option. However, for debugging 
purpose, I want all files in one directory to compile without -O2. However, 
Makefile in that directly takes CFLAGS from a parent Makefile which I can 
not modify !!

Let me know if there a some way to achieve this.

Thanks in advance.

-Mohan