gnu make : how to make conditional cflags
CFLAGS = -g -O0 -Wall -std=c99

CC =    gcc
CPP =   g++
LIBS =

PBDUMPER = pbdumper.exe

all: $(PBDUMPER)

$(PBDUMPER): pbdumper.c
    $(CC) $(CFLAGS) -o $@ $< $(LIBS)

clean:
    rm -f $(PBDUMPER)

.PHONY: clean

CFLAGS_COMMON = -std=c99
CFLAGS_DEBUG = -g -O0 -Wall
CFLAGS_RELEASE = -O2 -Wall
...
$(PBDUMPER): pbdumper.c
    $(CC) $(CFLAGS_COMMON) $(CFLAGS_DEBUG) -o $@ $< $(LIBS)

CFLAGS = -std=c99
# Release
# CFLAGS += -O2 -Wall
# Debug
CFLAGS += -g -O0 -Wall

ifeq ($(build),release)
    CFLAGS += -O2 -Wall
else
    CFLAGS += -g -O0 -Wall
endif

#debug is default, for another flavor : make BUILD=release
BUILD := debug

cflags.common := -std=c99
cflags.debug := -g -O0 -Wall
cflags.release := -O2 -Wall
CFLAGS := ${cflags.${BUILD}} ${cflags.common}
LIBS =

CC =    gcc
CPP =   g++

#executable
PBDUMPER = pbdumper.exe

all: $(PBDUMPER)

$(PBDUMPER): pbdumper.c
    $(CC) $(CFLAGS) -o $@ $< $(LIBS)

clean:
    rm -f $(PBDUMPER)

.PHONY: clean all