Dont use errno for error checking strtold

This leads to problems with the Xen build on travis.

Instead we perform the end pointer check and add a zero
length string check.

src/ctypes/ldouble_stubs.c

@@ -22,7 +22,6 @@
 #include <float.h>
 #include <math.h>
 #include <complex.h>
-#include <errno.h>
 
 #include "ctypes_ldouble_stubs.h"
 
@@ -373,11 +372,12 @@ CAMLprim value ctypes_ldouble_format(value width, value prec, value d) {
 CAMLprim value ctypes_ldouble_of_string(value v) {
   CAMLparam1(v);
   char *str = String_val(v);
-  char *end = str + caml_string_length(v);
+  int len = caml_string_length(v);
+  char *end;
   long double r;
-  errno = 0;
+  if (0 == len) caml_invalid_argument("LDouble.of_string");
   r = strtold(str, &end);
-  if (errno != 0 || *end != '\0') caml_invalid_argument("LDouble.of_string");
+  if (*end != '\0') caml_invalid_argument("LDouble.of_string");
   CAMLreturn(ctypes_copy_ldouble(r));
 }