Add compile flag to disable compilation of mining code

src/miner.h

@@ -23,8 +23,10 @@ struct CBlockTemplate
     std::vector<int64_t> vTxSigOps;
 };
 
+#ifdef ENABLE_MINING
 /** Run the miner threads */
 void GenerateBitcoins(bool fGenerate, CWallet* pwallet, int nThreads);
+#endif
 /** Generate a new block, without valid proof-of-work */
 CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn);
 CBlockTemplate* CreateNewBlockWithKey(CReserveKey& reservekey);