gnu make - How can default env variable in gnu makefile target -

i want this

test.install:         export build_number=${build_number:-$${another_var}} 

but not working. come blank

edit:

this works fine. provided export build_number in shell before invoking make command

test.install:         export build_number=${build_number} 

these not working . both give blank build_number

test.install:         export build_number=${build_number:-$${another_var}} 

and

test.install:         export build_number=${build_number:-55}} 

if want expand variables in bash rather in makefile, you'll need escape expansion otherwise make going variable literally called build_number:-${another_var} or build_number:-55

test.install:         export build_number=$${build_number:-$${another_var}}  test.install:         export build_number=$${build_number:-55}}