This code is from "http://dillingers.com/blog/2015/04/18/how-to-make-an-altcoin" with some changes for newer versions of Bitcoin. Use this for mine the genesis block for a new coin.

genesis.txt

        /**
        * This code is from "http://dillingers.com/blog/2015/04/18/how-to-make-an-altcoin" with some changes for newer versions of Bitcoin.
        * Use this for mine the genesis block for a new coin.
        * Put this before "consensus.hashGenesisBlock = genesis.GetHash();"
        */
        /////////GENESIS MINING///////////////
        /*
        printf("Genesis mining started\n");
        genesis.nNonce = 0;
        consensus.hashGenesisBlock = uint256S("0x001");
        for(genesis.nNonce == 0; UintToArith256(genesis.GetHash()) > UintToArith256(consensus.powLimit); genesis.nNonce++){  } 
        printf("New genesis merkle root: %s\n", genesis.hashMerkleRoot.ToString().c_str());
        printf("New genesis nonce: %lu\n", (unsigned long)genesis.nNonce);
        printf("New genesis hash: %s\n", genesis.GetHash().ToString().c_str());
        printf("Now replace the values, reComment the Genesis mining code and reCompile. \n");
        */
        //////////////////////////////////////

You forgot to add #include <arith_uint256.h>, otherwise you get errors error: ‘UintToArith256’ was not declared in this scope.