Changed makefile to use environment var LLVM_SRC_ROOT and not hard code the path

llvm/projects/visc-rt/Makefile

-LLVM_SRC_ROOT = /home/psrivas2/Hetero/VISC/Code/trunk/llvm
-LLVM_INSTALL = /home/psrivas2/Hetero/VISC/Code/trunk/llvm-install
+ifndef LLVM_SRC_ROOT
+  $(info "$$(LLVM_SRC_ROOT) is not defined")
+endif
+
+LLVM_INSTALL = $(LLVM_SRC_ROOT)/../llvm-install
 
 CPP_FLAGS = -I $(LLVM_SRC_ROOT)/include -I /usr/local/cuda/include
 TARGET:=visc-rt