Fixes to compile with g++ (GCC) 4.1.2 20080704 and Incisive 14.10

Fixes some issues with particular GCC and Incisive version, including compiler warnings and following Incisive  errors:

    -.--ns WARNING cocotb.gpi                                GpiCommon.cpp:60   in gpi_register_impl               VPI already registered, Check GPI_EXTRA
ERROR:          VPI       TFTHRE
         The user defined task or function $info has already been registered.
ERROR:          VPI       TFTHRE
         The user defined task or function $warning has already been registered.
ERROR:          VPI       TFTHRE
         The user defined task or function $error has already been registered.
ERROR:          VPI       TFTHRE
         The user defined task or function $fatal has already been registered.