Stop compilation if CUDALIB_INCLUDE_DIR not set when USE_POLLY_ACC==1

Author: singam-sanjay

src/Makefile

@@ -68,7 +68,7 @@ ifeq ($(USE_POLLY_ACC),1)
 LLVMLINK += -lPollyPPCG
 FLAGS += -DUSE_POLLY_ACC
 ifeq ($(CUDALIB_INCLUDE_DIR),)
-$(error CUDALIB_INCLUDE_DIR not set: Please provide the path to CUDA header files)
+$(error CUDALIB_INCLUDE_DIR not set: If compiling with USE_POLLY_ACC, user must provide the path to CUDA header files)
 endif
 FLAGS += -I$(CUDALIB_INCLUDE_DIR)
 FLAGS += -I$(shell $(LLVM_CONFIG_HOST) --src-root)/tools/polly/tools