Workaround for missing lconv support in android (#327)

fmt/format.h

@@ -910,6 +910,21 @@ class ThousandsSep {
   }
 };
 
+// Returns the thousands separator for the current locale.
+// On android the lconv structure is stubbed using an empty structure
+// The test is for the size only, not for the presense of 
+// thousands_sep in std::lconv, because if one would add thousands_sep
+// at some point, the size of structure would be at least sizeof(char*)
+template<typename Lconv, bool=(sizeof(Lconv) >= sizeof(char*))>
+struct Locale {
+  static fmt::StringRef thousands_sep() { return ""; }
+};
+
+template<typename Lconv>
+struct Locale<Lconv, true> {
+  static fmt::StringRef thousands_sep() { return static_cast<Lconv*>(std::localeconv())->thousands_sep; }
+};
+
 // Formats a decimal unsigned integer value writing into buffer.
 // thousands_sep is a functor that is called after writing each char to
 // add a thousands separator if necessary.
@@ -2778,7 +2793,8 @@ void BasicWriter<Char>::write_int(T value, Spec spec) {
   }
   case 'n': {
     unsigned num_digits = internal::count_digits(abs_value);
-    fmt::StringRef sep = std::localeconv()->thousands_sep;
+    fmt::StringRef sep =
+        internal::Locale<lconv>::thousands_sep();
     unsigned size = static_cast<unsigned>(
           num_digits + sep.size() * (num_digits - 1) / 3);
     CharPtr p = prepare_int_buffer(size, spec, prefix, prefix_size) + 1;