gmake: testing if a var is [un]defined

[Ernest]

Consider a variable called FOO...is there a more efficient/quick/simple way 
to test if this variable is defined (vs. undefined) at *runtime* than as 
follows?  Note that "defined" *can* mean an empty value ("FOO=").

# echo "no" if FOO is undefined
#  or "yes" if FOO is defined
foo:
   echo $(if $(findstring undefined,$(origin FOO)),no,yes)

Thanks,
Ernest