'-d:lto' flag breaks macOS
```
    NIM_CLI_ARGS = [
        '--opt:speed',
        '--parallelBuild:0',
        '--gc:refc',
        '--threads:on',
        '--app:lib',
        '-d:release',
        '-d:strip',
        # '-d:lto',
        '-d:ssl',
```

I have to comment out the above line to achieve build on macOS.

It's here: https://github.com/Pebaz/nimporter/blob/master/nimporter.py#L130

The root seems to be https://github.com/nim-lang/Nim/pull/15614

This issue is blocking https://github.com/juancarlospaco/faster-than-requests/issues/156

Maybe needs something like:
```
if not user supplied(--os: macOS or macOSX):
    NIM_CLI_ARGS += '-d:lto'
```

But there's a bigger issue: These inbuilt flags can't be removed by the user's .cfg file.

This makes the library brittle.

Need some way to 'RESET' these internal defaults from the .cfg file, maybe? e.g. `REMOVE='-d:lto'`...