VARIABLE = value
Normal setting of a variable, but any other variables mentioned with the
value field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared
VARIABLE := value
Setting of a variable with simple expansion of the values inside – values within it are expanded at declaration time.
Lazy Set If Absent
VARIABLE ?= value
Setting of a variable only if it doesn’t have a value.
value is always evaluated when
VARIABLE is accessed. It is equivalent to
ifeq ($(origin VARIABLE), undefined) VARIABLE = value endif
See the documentation for more details.
VARIABLE += value
Appending the supplied value to the existing value (or setting to that value if the variable didn’t exist)