user11262: I have been able to make a custom build of asmFish on Linux. However, I have been unable to do so on macOS. I've seen a Mac version of the FASM compiler on the Internet.  But when I tried using it for building it, it didn't generate a binary that could be run. 
I am running the latest macOS at time of writing. Any help appreciated. 

user17823: asmFish is NOT the same engine as Stockfish, so the answer by Dave is wrong. The difference is that asmFish is written in x86 Assembly (~20% faster), while Stockfish in C++.
Here is an issue I opened to test basically the time fasmg did to compile asmFish. It was closed, however, because the latest fasmg version fails to compile asmFish. The latest working version is this one.
You can download the version you want, but this is not necessary. You just need to open a command line and clone the repository:
git clone -b master https://github.com/lantonov/asmFish.git

Then, run cd asmFish to enter the directory and make.bat to compile.
For what it's worth:

There are already MacOS binaries in the main repository. They are in the MacOS_binaries folder.

There is also an archive of the executables in the executables branch, which is rarely updated.

You can directly compile asmFish from MacOS from the command line just having the whole x86 folder and the fasmg file:
 fasmg "x86\fish.asm" "asmFishX_type" -e 1000 -i "VERSION_OS='X'" -i "PEDANTIC = 1" -i "VERSION_POST = 'version'"



Replace type and version with the version post you want (base, popcnt, bmi1 and bmi2)

11262: <p>I have been able to make a custom build of asmFish on Linux. However, I have been unable to do so on macOS. I've seen a Mac version of the FASM compiler on the Internet.  But when I tried using it for building it, it didn't generate a binary that could be run. </p>

<p>I am running the latest macOS at time of writing. Any help appreciated. </p>
 17823: <p>asmFish is NOT the same engine as Stockfish, so the answer by Dave is wrong. The difference is that asmFish is written in x86 Assembly (~20% faster), while Stockfish in C++.</p>
<p><a href="https://github.com/lantonov/asmFish/issues/214" rel="nofollow noreferrer">Here</a> is an issue I opened to test basically the time fasmg did to compile asmFish. It was closed, however, because the latest fasmg version fails to compile asmFish. The latest working version is <a href="https://github.com/lantonov/asmFish/files/2418012/fasmg.zip" rel="nofollow noreferrer">this one</a>.</p>
<p>You can download the version you want, but this is not necessary. You just need to open a command line and clone the repository:</p>
<pre><code>git clone -b master https://github.com/lantonov/asmFish.git
</code></pre>
<p>Then, run <code>cd asmFish</code> to enter the directory and <code>make.bat</code> to compile.</p>
<p>For what it's worth:</p>
<ul>
<li><p>There are already MacOS binaries in the main repository. They are in the <code>MacOS_binaries</code> folder.</p>
</li>
<li><p>There is also an archive of the executables in the executables branch, which is rarely updated.</p>
</li>
<li><p>You can directly compile asmFish from MacOS from the command line just having the whole <code>x86</code> folder and the <code>fasmg</code> file:</p>
<pre><code> fasmg &quot;x86\fish.asm&quot; &quot;asmFishX_type&quot; -e 1000 -i &quot;VERSION_OS='X'&quot; -i &quot;PEDANTIC = 1&quot; -i &quot;VERSION_POST = 'version'&quot;
</code></pre>
</li>
</ul>
<p>Replace <code>type</code> and <code>version</code> with the version post you want (<code>base</code>, <code>popcnt</code>, <code>bmi1</code> and <code>bmi2</code>)</p>
