I believe Prjtrellis has CMAKE define pytrellis as a shared module, not library.
On MacOS, this makes a difference and NextPNR expected ``.dylib``
instead of ``.so``. Things still work on Linux.
`if(NOT DEFINED)` is not appropriate since a variable that contains
`-NOTFOUND` still counts as `DEFINED`. This can cause issues if
configuration fails, writes `-NOTFOUND` to the cache, and is then
restarted.
