Convert USE_READLINE config option to be consistent with others.

unix/Makefile

@@ -2,7 +2,7 @@ PYSRC=../py
 BUILD=build
 
 CC = gcc
-CFLAGS = -I. -I$(PYSRC) -Wall -Werror -ansi -std=gnu99 -Os -DUSE_READLINE #-DNDEBUG
+CFLAGS = -I. -I$(PYSRC) -Wall -Werror -ansi -std=gnu99 -Os #-DNDEBUG
 LDFLAGS = -lm
 
 SRC_C = \

unix/main.c

@@ -15,7 +15,7 @@
 #include "runtime.h"
 #include "repl.h"
 
-#ifdef USE_READLINE
+#if MICROPY_USE_READLINE
 #include <readline/readline.h>
 #include <readline/history.h>
 #endif
@@ -35,7 +35,7 @@ static char *str_join(const char *s1, int sep_char, const char *s2) {
 }
 
 static char *prompt(char *p) {
-#ifdef USE_READLINE
+#if MICROPY_USE_READLINE
     char *line = readline(p);
     if (line) {
         add_history(line);

unix/mpconfigport.h

@@ -1,5 +1,10 @@
 // options to control how Micro Python is built
 
+// Linking with GNU readline causes binary to be licensed under GPL
+#ifndef MICROPY_USE_READLINE
+#define MICROPY_USE_READLINE        (1)
+#endif
+
 #define MICROPY_ENABLE_FLOAT        (1)
 #define MICROPY_EMIT_CPYTHON        (0)
 #define MICROPY_EMIT_X64            (1)