user2306: According to the bitcoin wiki:

The first 50BTC block reward went to address 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa, though this reward can't be spent due to a quirk in the way that the genesis block is expressed in the code (this may have been intentional).

But where is this quirk? How would you change it to allow it to be spent?

user14887: At least with current versions of the Bitcoin Core client, the Genesis block is actually hard coded into the client.
The part of the code that builds this block is located here and I have included a JSON dump of the coinbase transaction below for reference.
{
    "txid" : "4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b",
    "version" : 1,
    "locktime" : 0,
    "vin" : [
        {
            "coinbase" : "04ffff001d0104455468652054696d65732030332f4a616e2f32303039204368616e63656c6c6f72206f6e206272696e6b206f66207365636f6e64206261696c6f757420666f722062616e6b73",
            "sequence" : 4294967295
        }
    ],
    "vout" : [
        {
            "value" : 50.00000000,
            "n" : 0,
            "scriptPubKey" : {
                "asm" : "04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5f OP_CHECKSIG",
                "hex" : "4104678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5fac",
                "reqSigs" : 1,
                "type" : "pubkey",
                "addresses" : [
                    "1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa"
                ]
            }
        }
    ]
}

Reference Gist

user60443: 
How would you change it to allow it to be spent?

In order to make the coinbase spendable, the following changes have to be made to validation.cpp (v0.16.2). Note: as mentioned this would be considered a hard fork.
Step 1 - Disable skipping the genesis block (which is intended to keep the bug as part of consensus), Line 1818:  
// Special case for the genesis block, skipping connection of its transactions
    // (its coinbase is unspendable)
    if (block.GetHash() == chainparams.GetConsensus().hashGenesisBlock) {
        if (!fJustCheck)
            view.SetBestBlock(pindex->GetBlockHash());
        // return true; <- comment this line out
}

Step 2 - Skip assertion of previous block for genesis block, Line 1874:
if (block.GetHash() != chainparams.GetConsensus().hashGenesisBlock) {
    assert(pindex->pprev);
}

Step 3 - Skip writing undo data for genesis block, Line 1989:
if (block.GetHash() != chainparams.GetConsensus().hashGenesisBlock)
{
    if (!WriteUndoDataForBlock(blockundo, state, pindex, chainparams))
        return false;
} 


user4070: When a node starts up it initializes its copy of the block database alongside the genesis block and then begins the synchronization process. For some reason, Satoshi decided not to add the coinbase transaction from the genesis block to the global transaction database. Thus all the nodes in the network would reject the block.  
I'm not sure if this was done on purpose or if it was simply an oversight. In any event, it is forever bound to 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa. 

2306: <p>According to the <a href="https://en.bitcoin.it/wiki/Genesis_block">bitcoin wiki</a>:</p>

<blockquote>
  <p>The first 50BTC block reward went to address <a href="http://blockexplorer.com/address/1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa">1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa</a>, though this reward can't be spent due to a quirk in the way that the genesis block is expressed in the code (this may have been intentional).</p>
</blockquote>

<p>But where is this quirk? How would you change it to allow it to be spent?</p>
 14887: <p>At least with current versions of the Bitcoin Core client, the Genesis block is actually hard coded into the client.</p>

<p>The part of the code that builds this block is located <a href="https://gist.github.com/drazisil/8f628627cff0d173e9b1" rel="nofollow noreferrer">here</a> and I have included a JSON dump of the coinbase transaction below for reference.</p>

<pre><code>{
    "txid" : "4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b",
    "version" : 1,
    "locktime" : 0,
    "vin" : [
        {
            "coinbase" : "04ffff001d0104455468652054696d65732030332f4a616e2f32303039204368616e63656c6c6f72206f6e206272696e6b206f66207365636f6e64206261696c6f757420666f722062616e6b73",
            "sequence" : 4294967295
        }
    ],
    "vout" : [
        {
            "value" : 50.00000000,
            "n" : 0,
            "scriptPubKey" : {
                "asm" : "04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5f OP_CHECKSIG",
                "hex" : "4104678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5fac",
                "reqSigs" : 1,
                "type" : "pubkey",
                "addresses" : [
                    "1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa"
                ]
            }
        }
    ]
}
</code></pre>

<p>Reference <a href="https://gist.github.com/drazisil/8f628627cff0d173e9b1" rel="nofollow noreferrer">Gist</a></p>
 60443: <blockquote>
  <p>How would you change it to allow it to be spent?</p>
</blockquote>

<p>In order to make the coinbase spendable, the following changes have to be made to validation.cpp (v0.16.2). <strong>Note</strong>: as mentioned this would be considered a hard fork.</p>

<p><strong>Step 1</strong> - Disable skipping the genesis block (which is intended to keep the bug as part of consensus), <a href="https://github.com/bitcoin/bitcoin/blob/v0.16.2/src/validation.cpp#L1818" rel="noreferrer">Line 1818</a>:  </p>

<pre><code>// Special case for the genesis block, skipping connection of its transactions
    // (its coinbase is unspendable)
    if (block.GetHash() == chainparams.GetConsensus().hashGenesisBlock) {
        if (!fJustCheck)
            view.SetBestBlock(pindex-&gt;GetBlockHash());
        // return true; &lt;- comment this line out
}
</code></pre>

<p><strong>Step 2</strong> - Skip assertion of previous block for genesis block, <a href="https://github.com/bitcoin/bitcoin/blob/v0.16.2/src/validation.cpp#L1874" rel="noreferrer">Line 1874</a>:</p>

<pre><code>if (block.GetHash() != chainparams.GetConsensus().hashGenesisBlock) {
    assert(pindex-&gt;pprev);
}
</code></pre>

<p><strong>Step 3</strong> - Skip writing undo data for genesis block, <a href="https://github.com/bitcoin/bitcoin/blob/v0.16.2/src/validation.cpp#L1989" rel="noreferrer">Line 1989</a>:</p>

<pre><code>if (block.GetHash() != chainparams.GetConsensus().hashGenesisBlock)
{
    if (!WriteUndoDataForBlock(blockundo, state, pindex, chainparams))
        return false;
} 
</code></pre>
 4070: <p>When a node starts up it initializes its copy of the block database alongside the genesis block and then begins the synchronization process. For some reason, Satoshi decided not to add the coinbase transaction from the genesis block to the global transaction database. Thus all the nodes in the network would reject the block.  </p>

<p>I'm not sure if this was done on purpose or if it was simply an oversight. In any event, it is forever bound to 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa. </p>
