Hey folks! 
Just wanted to share a quick fix that saved me a ton of time while building Unreal Engine from source on Linux.
I was running into a super annoying ISPC-related error during the engine build process. I saw lots of forum posts and GitHub issues, but most of them were about compiling game projects — not the engine itself. So it took a while to figure out the right solution.
After digging through some of the build files, I found the culprit. The fix was surprisingly simple:
In this file:
Engine/Source/Programs/UnrealBuildTool/Platform/Linux/UEBuildLinux.cs
Around line 273, change this line:
Target.bCompileISPC = true;
to:
Target.bCompileISPC = false;
And that’s it! The engine compiled successfully after this change, and everything’s been working fine since. 
If you’re struggling with weird ISPC toolchain errors on Linux (especially if they mention missing includes, compiler errors, or internal tool crashes), this might help you too.
Hope this saves someone else a few hours of frustration! 
Feel free to reply if you’re still stuck — I might not remember every detail, but happy to help if I can.
Cheers,
BadiCo