tools/os_support: add OS X Lion support

OS X Lion's c-library implements getline(), therefore prevent including the old
helper implementation for __DARWIN_C_LEVEL < 200809L.

Without this patch following error occours:

---8<---
In file included from os_support.h:32,
                 from img2srec.c:55:
getline.h:1: error: conflicting types for ‘getline’
/usr/include/stdio.h:449: error: previous declaration of ‘getline’ was
here
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>

tools/os_support.c

@@ -23,6 +23,6 @@
 #ifdef __MINGW32__
 #include "mingw_support.c"
 #endif
-#ifdef __APPLE__
+#if defined(__APPLE__) && __DARWIN_C_LEVEL < 200809L
 #include "getline.c"
 #endif

tools/os_support.h

@@ -28,7 +28,7 @@
 #include "mingw_support.h"
 #endif
 
-#ifdef __APPLE__
+#if defined(__APPLE__) && __DARWIN_C_LEVEL < 200809L
 #include "getline.h"
 #endif