CMake: Don't include Abseil if it is not used.
CMake: Don't include Abseil if it is not used.

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