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GNU make (and many others) has two main ways of assigning values to variables. They differ according to the operator which you use. According to the documentation, a single equals sign (=) will cause a recursive expansion of the value, whereas a colon-equals (:=) will cause a simple expansion.
Your quoted code uses a := and so should cause a simple expansion. What you are seeing is an error message associated with a recursive expansion. I would expect that sort of error if you had something like this:
PATH = $(PATH):/other/dir
Could the error be being caused by a different line in your makefile which you haven't quoted? If you're sure that your cut-and-pasting is correct, and that it is this line which is causing the problem, it would be helpful if we could see the whole, unedited makefile.