unix: Fix linker errors when time/ffi modules are disabled

When disabling these via mpconfigport.mk or on the commandline,
the correspoding build options are not set and the sources are not
built so the modules should not be added to the
MICROPY_EXTRA_BUILTIN_MODULES list since they are undefined.

unix/mpconfigport.h

@@ -52,9 +52,21 @@
 extern const struct _mp_obj_module_t mp_module_time;
 extern const struct _mp_obj_module_t mp_module_socket;
 extern const struct _mp_obj_module_t mp_module_ffi;
+
+#if MICROPY_MOD_FFI
+#define MICROPY_MOD_FFI_DEF { MP_OBJ_NEW_QSTR(MP_QSTR_ffi), (mp_obj_t)&mp_module_ffi },
+#else
+#define MICROPY_MOD_FFI_DEF
+#endif
+#if MICROPY_MOD_TIME
+#define MICROPY_MOD_TIME_DEF { MP_OBJ_NEW_QSTR(MP_QSTR_time), (mp_obj_t)&mp_module_time },
+#else
+#define MICROPY_MOD_TIME_DEF
+#endif
+
 #define MICROPY_EXTRA_BUILTIN_MODULES \
-    { MP_OBJ_NEW_QSTR(MP_QSTR_ffi), (mp_obj_t)&mp_module_ffi }, \
-    { MP_OBJ_NEW_QSTR(MP_QSTR_time), (mp_obj_t)&mp_module_time }, \
+    MICROPY_MOD_FFI_DEF \
+    MICROPY_MOD_TIME_DEF \
     { MP_OBJ_NEW_QSTR(MP_QSTR_microsocket), (mp_obj_t)&mp_module_socket }, \
 
 // type definitions for the specific machine