Adapt sources to configurable config.h name
diff --git a/library/arc4.c b/library/arc4.c
index 536ea8c..dbf0c7e 100644
--- a/library/arc4.c
+++ b/library/arc4.c
@@ -28,7 +28,11 @@
  *  http://groups.google.com/group/sci.crypt/msg/10a300c9d21afca0
  */
 
+#if !defined(POLARSSL_CONFIG_FILE)
 #include "polarssl/config.h"
+#else
+#include POLARSSL_CONFIG_FILE
+#endif
 
 #if defined(POLARSSL_ARC4_C)