Added mechanism to provide alternative cipher / hash implementations

All symmetric cipher algorithms and hash algorithms now include support
for a POLARSSL_XXX_ALT flag that prevents the definition of the
algorithm context structure and all 'core' functions.
(cherry picked from commit 4087c47043)

library/md4.c

@@ -1,7 +1,7 @@
 /*
  *  RFC 1186/1320 compliant MD4 implementation
  *
- *  Copyright (C) 2006-2010, Brainspark B.V.
+ *  Copyright (C) 2006-2013, Brainspark B.V.
  *
  *  This file is part of PolarSSL (http://www.polarssl.org)
  *  Lead Maintainer: Paul Bakker <polarssl_maintainer at polarssl.org>
@@ -39,6 +39,8 @@
 #include <stdio.h>
 #endif
 
+#if !defined(POLARSSL_MD4_ALT)
+
 /*
  * 32-bit integer manipulation macros (little endian)
  */
@@ -259,6 +261,8 @@ void md4_finish( md4_context *ctx, unsigned char output[16] )
     PUT_UINT32_LE( ctx->state[3], output, 12 );
 }
 
+#endif /* !POLARSSL_MD4_ALT */
+
 /*
  * output = MD4( input buffer )
  */