simulator/verilator: add defines_files directory to include_dirs

Without this, the automatically generated define header file
can't be found by the verilator source files.
section/verilator: add $(SYSTEMC)/include to include dirs

Also, only add the SYSTEMC_INCLUDE environment variable to
the include dirs if it's set.
section/verilator: fix build_SysC launcher

The sources can be found in src_root/core/src_file, update
the build_SysC launcher to reflect this.