sc_externs.h signatures do not match standard. add on extern "C"

[mattgately]

sc_externs.h forward declares the following functions:

extern "C" int sc_main( int argc, char* argv[] );

namespace sc_core {
	extern "C" int sc_elab_and_sim( int argc, char* argv[] );
	extern "C" int sc_argc();
	extern "C" const char* const* sc_argv();

It appends extern "C" to the signature of the functions, which is not compatible with declarations of these functions in the SystemC LRM sections 4.3.1-4.3.4.

Another source file that declares and defines the functions according to the standard, without extern "C" is unable to link to the systemc library correctly.

[maehne]

Thanks @mattgately for reporting this issue! The LWG will have a look at it.