Don't define strnlen in MacOS >= 10.7

In MacOS 10.7, strnlen is already available.

Change-Id: If95bdde2831200026e04fe266bfed697c384fd34
diff --git a/config-compat-darwin.h b/config-compat-darwin.h
index 7953bb6..f884b09 100644
--- a/config-compat-darwin.h
+++ b/config-compat-darwin.h
@@ -38,6 +38,7 @@
 	return NULL;
 }
 
+#if __MAC_OS_X_VERSION_MIN_REQUIRED < 1070
 static inline size_t strnlen (const char *__string, size_t __maxlen)
 {
 	int len = 0;
@@ -45,6 +46,7 @@
 		len++;
 	return len;
 }
+#endif
 
 static inline void *mempcpy (void * __dest, const void * __src, size_t __n)
 {