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Got it! Then modify the make file to include the object of the class which has the chainActive as a member. e.g 'x.o' where chainActive is a member of class 'x'
This is due to missing the object of the class which has 'chainActive()' method. So including which ever the file that has the method. In case if it gives further different undefined reference errors; find the file that contains the method; include it; until it compiles ;)