message: In config/defaults.mk, use '=' to set {OCCA,RAJA}_DIR.
  This makes it easier to configure mfem by copying defaults.mk to
  user.mk and editting it: if using '?=', the value given in user.mk
  will not overwrite the one from defaults.mk.
diff: @@ -307,12 +307,12 @@ CUDA_OPT =
  CUDA_LIB =
  # OCCA library configuration
  -OCCA_DIR ?= @MFEM_DIR@/../occa
  +OCCA_DIR = @MFEM_DIR@/../occa
  OCCA_OPT = -I$(OCCA_DIR)/include
  OCCA_LIB = $(XLINKER)-rpath,$(OCCA_DIR)/lib -L$(OCCA_DIR)/lib -locca
  # RAJA library configuration
  -RAJA_DIR ?= @MFEM_DIR@/../raja
  +RAJA_DIR = @MFEM_DIR@/../raja
  RAJA_OPT = -I$(RAJA_DIR)/include
  ifdef CUB_DIR
  RAJA_OPT += -I$(CUB_DIR)
  