c++ - Cannot compile SDL with Makefile -

my makefile consists of following:

cc= g++ libs= -lsdl  clean:     rm  -f  

it won't compile because g++ not take -lsdl parameter though included it.

when g++ a.cpp -o -lsdl directly in terminal compiles fine, though.

add rule:

a: a.cpp     $(cc) $< -o $@ $(libs)