CMake: Set PORTFOLIO_BUILD when building pcvc4 (#2666)

CMake: Set PORTFOLIO_BUILD when building pcvc4 (#2666)

Back when we used Autotools, we set the PORTFOLIO_BUILD macro when
building pcvc4. Our CMake build system is currently not doing that, so
setting thread options when running pcvc4 results in an error message
saying that "thread options cannot be used with sequential CVC4."
This commit fixes that behavior by recompiling driver_unified.cpp with
different options for the cvc4 and the pcvc4 binary.

[0] https://github.com/CVC4/CVC4/blob/7de0540252b62080ee9f98617f5718cb1ae08579/src/main/Makefile.am#L36