Append to GNU ‘make’ variables via the command line

Check out the override directive. You will probably need to modify the makefile once, but it should do what you want.

Example makefile:

override CFLAGS += -Wall

app: main.c
    gcc $(CFLAGS) -o app main.c 

Example command lines:

$ make
gcc -Wall -o app main.c 
$ make CFLAGS=-g
gcc -g -Wall -o app main.c