sim/verilator: check for SYSTEMC_INCLUDE and SYSTEMC_LIBDIR

Before we use SystemC, we should check for SYSTEMC_INCLUDE
and SYSTEMC_LIBDIR environment variable so we don't have
to read the log file to see what was wrong.

Those two values can be set in verilator at compile time.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>