Compatibility with gcc-4.3

Add include statements (<cstdlib> for abort()) and a cast.
Necessary and sufficient for successful compilation with
gcc-4.3 (Debian 4.3.0-4) 4.3.1 20080501 (prerelease)

elab_scope.cc

@@ -22,6 +22,7 @@
 # include  "netmisc.h"
 # include  <cstring>
 # include  <iostream>
+# include  <cstdlib>
 # include  <stdio.h>
 
 /*

net_design.cc

@@ -20,6 +20,7 @@
 # include "config.h"
 
 # include  <iostream>
+# include  <cstdlib>
 
 /*
  * This source file contains all the implementations of the Design

net_scope.cc

@@ -22,6 +22,7 @@
 
 # include  "netlist.h"
 # include  <cstring>
+# include  <cstdlib>
 # include  <sstream>
 # include  "ivl_assert.h"
 
@@ -218,7 +219,7 @@ map<perm_string,NetScope::param_expr_t>::iterator NetScope::find_parameter(perm_
       if (idx != localparams.end())
 	    return idx;
 
-      return 0;
+      return (map<perm_string,param_expr_t>::iterator) 0;
 }
 
 NetScope::TYPE NetScope::type() const