This info copied from: https://vuanhduy.wordpress.com/2014/05/02/the-variants-of-gnu-makefile-assignment-operation/

The differences in =, :=, ?= and += when assigning a macro in Makefile.

= operation

If the variable does not have a value, the operation assign the value to it
If the variable already had a value, it is replaced.
This value will be expanded when […]