ARROW-8000: [C++] Fix compilation on gcc 4.8

It seems that `constexpr auto` with function pointers is not supported in gcc 4.8.

Closes #6538 from wesm/ARROW-8000-gcc48-fixes and squashes the following commits:

ca01694 <Wes McKinney> Fix compilation on gcc 4.8

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

cpp/src/arrow/c/bridge_test.cc

@@ -45,21 +45,28 @@ using internal::SchemaExportTraits;
 template <typename T>
 struct ExportTraits {};
 
+template <typename T>
+using Exporter = std::function<Status(const T&, struct ArrowSchema*)>;
+
 template <>
 struct ExportTraits<DataType> {
-  static constexpr auto ExportFunc = ExportType;
+  static Exporter<DataType> ExportFunc;
 };
 
 template <>
 struct ExportTraits<Field> {
-  static constexpr auto ExportFunc = ExportField;
+  static Exporter<Field> ExportFunc;
 };
 
 template <>
 struct ExportTraits<Schema> {
-  static constexpr auto ExportFunc = ExportSchema;
+  static Exporter<Schema> ExportFunc;
 };
 
+Exporter<DataType> ExportTraits<DataType>::ExportFunc = ExportType;
+Exporter<Field> ExportTraits<Field>::ExportFunc = ExportField;
+Exporter<Schema> ExportTraits<Schema>::ExportFunc = ExportSchema;
+
 // An interceptor that checks whether a release callback was called.
 // (for import tests)
 template <typename Traits>