Check for `LLVM_VERSION_MAJOR >= 18`

src/llvm_backend.cpp

@@ -3082,9 +3082,11 @@ gb_internal bool lb_generate_code(lbGenerator *gen) {
 		m->target_machine = target_machine;
 		LLVMSetModuleDataLayout(m->mod, LLVMCreateTargetDataLayout(target_machine));
 
+	#if LLVM_VERSION_MAJOR >= 18
 		if (build_context.fast_isel) {
 			LLVMSetTargetMachineFastISel(m->target_machine, true);
 		}
+	#endif
 
 		array_add(&target_machines, target_machine);
 	}