pb:/tmp> cat Makefile
testing:
        @echo "extern : myvar='$(myvar)'"
        @echo "default: CPP='$(CPP)'"

pb:/tmp> make
extern : myvar=''
default: CPP='cc -E'

pb:/tmp> make myvar=Tim\ Towdy
extern : myvar='Tim Towdy'
default: CPP='cc -E'

pb:/tmp> make myvar=gcc  CPP='$(myvar)'
extern : myvar='gcc'
default: CPP='gcc'