This allows Gem5 to be build with SystemC kernel when build with 'scons sysc_build/*'.

NOTE: There is a small bug with SystemC 2.3.0 which is the following:

    systemc-2.3.0/include/sysc/communication/sc_interface.h:72: error: "__SUNPRO_CC" is not defined

and can be fixed by replacing

private:
	static sc_event m_never_notified;
#if __SUNPRO_CC == 0x520
	// Workaround for a bug in the Sun WorkShop 6 update 2 compiler.
	// An empty virtual base class can cause the optimizer to
	// generate wrong code.
	char dummy;
#endif
};

by

private:
	static sc_event m_never_notified;
#if defined(__SUNPRO_CC) && (__SUNPRO_CC == 0x520)
	// Workaround for a bug in the Sun WorkShop 6 update 2 compiler.
	// An empty virtual base class can cause the optimizer to
	// generate wrong code.
	char dummy;
#endif
};