Hack: silence ignored system() result with pragma.

The third and last of three branches aimed at reducing gcc warnings.

This one takes a rather hackish approach: the system() call, marked as warn_unused_result in gcc, is called inside an inline function within a pragma block disabling the warning.

This preserves the current ACADO behaviour while silencing the warnings.  However, it might be better to throw an exception on a non-zero return value.