unix/main: Only accept full emit cmd-line options if native enabled.

ports/unix/main.c

@@ -313,7 +313,11 @@ STATIC int usage(char **argv) {
     int impl_opts_cnt = 0;
     printf(
 "  compile-only                 -- parse and compile only\n"
+#if MICROPY_EMIT_NATIVE
 "  emit={bytecode,native,viper} -- set the default code emitter\n"
+#else
+"  emit=bytecode                -- set the default code emitter\n"
+#endif
 );
     impl_opts_cnt++;
 #if MICROPY_ENABLE_GC
@@ -343,10 +347,12 @@ STATIC void pre_process_options(int argc, char **argv) {
                     compile_only = true;
                 } else if (strcmp(argv[a + 1], "emit=bytecode") == 0) {
                     emit_opt = MP_EMIT_OPT_BYTECODE;
+                #if MICROPY_EMIT_NATIVE
                 } else if (strcmp(argv[a + 1], "emit=native") == 0) {
                     emit_opt = MP_EMIT_OPT_NATIVE_PYTHON;
                 } else if (strcmp(argv[a + 1], "emit=viper") == 0) {
                     emit_opt = MP_EMIT_OPT_VIPER;
+                #endif
 #if MICROPY_ENABLE_GC
                 } else if (strncmp(argv[a + 1], "heapsize=", sizeof("heapsize=") - 1) == 0) {
                     char *end;