Compatibility with gcc-4.3

Add "include"s to fix errors flagged by gcc-4.3.2

elab_expr.cc

@@ -21,6 +21,7 @@
 # include  <typeinfo>
 # include  <cstdlib>
 # include  <cstring>
+# include  <limits.h>
 # include "compiler.h"
 
 # include  "pform.h"

expr_synth.cc

@@ -20,6 +20,7 @@
 # include "config.h"
 # include "compiler.h"
 
+# include  <stdlib.h>
 # include  <iostream>
 
 # include  "netlist.h"

tgt-null/null.c

@@ -18,6 +18,7 @@
  */
 
 # include "config.h"
+# include  <string.h>
 
 /*
  * This is a null target module. It does nothing.