@Ken -

Function calls aren't governed under GPL rules, else ALL computer software would run into the statement you made there. All software makes function calls to the software below it, making it dependent on the code, but NOT derivative.

In the Galoob vs. Nintendo court case, the courts made that point very explicitly clear:
Some time ago, for example, computer companies began marketing spell-checkers that operate within existing word processors by signalling the writer when a word is misspelled. These applications, as well as countless others, could not be produced and marketed if courts were to conclude that the word processor and spell-checker combination is a derivative work based on the word processor alone.