makefiles - accumulate c files during once

i wish examination gcc whole way optimizations. i have pass c-files during once compiler frontend. however, i makefiles automate build process, i'm an consultant comes makefile magic.

how should i cgange makefile i wish accumulate (maybe even link) controlling only gcc?

for anxiety - makefile looks this:

libs  = -lkernel32 -luser32 -lgdi32 -lopengl32
cflags = -wall

obj = 64bitmath.o    \
      monotone.o     \
      node_sort.o    \
      planesweep.o   \
      triangulate.o  \
      prim_combine.o \
      welding.o      \
      test.o         \
      main.o

%.o : %.c
    gcc -c $(cflags) $< -o $@

test: $(obj)
    gcc -o $@ $^ $(cflags) $(libs)