add a conditional assignment operator ?=

[svark]

Make provides a conditional operator ?=

var ?= value

which sets a value for var provided var is empty. Please provide such an operator in tupfiles too. This would allow less special case treatment when porting makefiles to tupfiles.

[typeless]

Happened to need it too. It's not obvious to me at first, but

ifeq ($(FOO), "")
FOO = yes
endif 

can do it. I thought that was ifndef, but ifndef is for @-variable only.