const char* for ""

Index: main.cpp
--- main.cpp.orig
+++ main.cpp
@@ -66,7 +66,7 @@ int verbose = 0;
 int main(int argc, char *argv[])
 try
 {
-  char c;
+  int c;
   int option_index = 0;
   
   gfx_int *iface = 0;
@@ -74,8 +74,8 @@ try
   ostream *ofile = 0;
   
   char *outfile_str = 0;
-  char *interpreter_str = "turtle";
-  char *interface_str = "gl";
+  const char *interpreter_str = "turtle";
+  const char *interface_str = "gl";
   char *module_str = 0;
   double min, max, step;
   int show_depth = 0;