Allow enabling or disabling readline support by setting HAVE_READLINE.

Makefile

@@ -23,9 +23,8 @@ endif
 
 ifeq "$(shell uname)" "Linux"
   CFLAGS += -ffunction-sections -fdata-sections
-  CFLAGS += -DHAVE_READLINE
   LDFLAGS += -Wl,--gc-sections
-  LIBREADLINE += -lreadline
+  HAVE_READLINE := yes
 endif
 
 ifeq "$(build)" "debug"
@@ -38,6 +37,11 @@ else
   LDFLAGS += -Wl,-s
 endif
 
+ifeq "$(HAVE_READLINE)" "yes"
+  CFLAGS += -DHAVE_READLINE
+  LIBREADLINE += -lreadline
+endif
+
 CFLAGS += $(XCFLAGS)
 
 # You shouldn't need to edit anything below here.