After tinkering with this a bit more, it doesn't look like I need to set the environment variable, but it would give me more details.
It's on my to-do list now, thanks guys!
No frameworks or fancy setup, I'm just cross compiling from my Mac set to a Linux x64 target, then zipping and uploading the executable, the whole process takes seconds.
And here's the result of running a pure FPC custom runtime on AWS lambda.
Init Duration: 13.68 ms
Duration: 1.58 ms
Billed Duration: 100 ms
Memory Size: 128 MB
Max Memory Used: 31 MB
Max memory is for the entire container, this little runtime doesn't do anything other than echo back whatever input it got.
For anyone else using AWS lambdas, that duration is with the lambda being warm, though cold starts are crazy fast with FPC on lambda anyway.
Next up, figuring out how to talk to AWS databases.
This will be an article series once I document the process, it really is smooth and without any of the hackery.
No side-loading, no launching through Node.js, etc. Just a compiled FPC program doing its thing.