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>