bnfc has its own logic of preserving unchanged files that makes it incompatible with make. GNU make expects a target to get the current time stamp if the action is run that creates it or refreshes it from its dependencies. Since bnfc is "clever" and does not recreate unchanged files, it will be called every time by make should there be any target older than the .cf file.

This contradicts the principle that make should be idempotent. More precisely, the second call to make in a row should report nothing to do for a properly written Makefile.