Generic handling of `when` boolean expressions

[DanielGavin]

Right now the when conditions are hardcoded for handling ODIN_OS, but at some point it would be nice to support many other variables of the ODIN_* prefixes, and also not have it hardcoded, i.e. right now it only detects when ODIN_OS == "windows", but not when "windows" == ODIN_OS