Use HIP_PATH if set else use relative ../...

[ROCm/hip commit: 40e72dcd4a]

projects/hip/samples/0_Intro/square/Makefile

@@ -1,4 +1,4 @@
-HIP_PATH=../../..
+HIP_PATH?=../../..
 HIPCC=$(HIP_PATH)/bin/hipcc
 
 all: square.hip.out