Exercise using Dune's virtual libraries and conditional compilation.
We want to have two different implementations of system_process, one for Linux and another for MacOS.
These can be included in conditionally compiled executables. However trying to use them in tests results in this error:
$ dune build @all
File "test/dune", line 5, characters 20-48:
5 | (libraries virtual virtual.linux_system_process))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: Library "virtual.linux_system_process" in
_build/default/lib/system_process/linux is hidden (unsatisfied 'enabled_if').
-> required by _build/default/test/test_virtual.exe
-> required by alias test/all
It's unclear how to use virtual libraries in test code.