In GNU make, you can use Target-specific Variable Values (https://www.gnu.org/software/make/manual/make.html#Target_00...)

You can also do

  DEBUG ?= 1
  ifeq ($(DEBUG), 1)
    CFLAGS =<whatever you want>
  else
    CFLAGS=<whatever you want>
  endif

and then build with

  make DEBUG=0

for the release build.

In BSD make, you can use something like

  .ifmake target
  CFLAGS := <whatever you want>
  .else
  CFLAGS := <whatever you want>
  .endif