Fix error introduced in Fl_arg.cxx when converted to the driver model.

fluid built by CMake no longer started under the finder.

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3-porting@11521 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

src/Fl_arg.cxx

@@ -159,6 +159,7 @@ int Fl::arg(int argc, char **argv, int &i) {
     return 1;
   } else if (Fl::system_driver()->single_arg(s)) {
     i++;
+    return 1;
   }
 
   const char *v = argv[i+1];