See [this example](https://github.com/unicredit/linear-algebra/blob/master/linalg.nim#L17-L18) where I give an error on JS backend: when defined(js): {.fatal: "linalg is only available for native backends".}
I assume there should be similar flags for `cpp` and so on, but I am not sure where to find the exact list (there should be also Objective C and PHP?)
