Update CMakeLists.txt
Update CMakeLists.txt

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.
SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr