Made the Makefile respect environment variable LLVM_DIR
Made the Makefile respect environment variable LLVM_DIR


Former-commit-id: e014d2aadde6043032d253b759874a8a650db13c
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide