Fix $(BUILD) -> $(BUILD_DIR) in k_model_utils.c rule.

Fix $(BUILD) -> $(BUILD_DIR) in k_model_utils.c rule.

BUILD isn't defined, and the old way gave errors like:

  $ make -j17 all_bin
  mkdir build
  notangle -Rk-model-utils.c src/sawsim.nw > build/k_model_utils.c
  /bin/sh: cannot create build/k_model_utils.c: Directory nonexistent
  make: *** [build/k_model_utils.c] Error 2
  make: *** Waiting for unfinished jobs....

You'd only have seen this error with parallel builds, otherwise
BUILD_DIR would have already have been created as a dependency of
another file by the time k_model_utils.c needed it.