Fix dependency file (-M) generation.

The dependency file may be written by ivlpp as it loads its own
dependencies. Make sure this doesn't mess up the dependencies that
are already written by the main program. This requires that ivl,
ivlpp and iverilog (driver) cooperate on the opening of the
dependency file.

load_module.cc

@@ -73,6 +73,7 @@ bool load_module(const char*type)
 
 	    if(depend_file) {
 		  fprintf(depend_file, "%s\n", path);
+		  fflush(depend_file);
 	    }
 
 	    if (ivlpp_string) {