Add a grisu test stub

src/format.cc

@@ -10,6 +10,10 @@
 FMT_BEGIN_NAMESPACE
 template struct internal::basic_data<void>;
 
+// Workaround a bug in MSVC2013 that prevents instantiation of grisu2_format.
+bool (*instantiate_grisu2_format)(double, internal::buffer&,
+                                  core_format_specs) = internal::grisu2_format;
+
 #ifndef FMT_STATIC_THOUSANDS_SEPARATOR
 template FMT_API internal::locale_ref::locale_ref(const std::locale& loc);
 template FMT_API std::locale internal::locale_ref::get<std::locale>() const;