Remove '=delete' from template methods for Xcode 8 (#1133)

For Apple clang-800.0.42.1, which was released with Xcode 8 in
September 2016, the '=delete' on the 'is' and 'as' methods causes
the following errors for value.h:

  inline declaration of 'as<bool>' follows non-inline definition
  inline declaration of 'is<bool>' follows non-inline definition

etcetera for the other specializations of 'is' and 'as'.  The same
problem also occurs for clang-3.8 but not clang-3.9 or later.

include/json/value.h

@@ -21,6 +21,24 @@
 #endif
 #endif
 
+// Support for '= delete' with template declarations was a late addition
+// to the c++11 standard and is rejected by clang 3.8 and Apple clang 8.2
+// even though these declare themselves to be c++11 compilers.
+#if !defined(JSONCPP_TEMPLATE_DELETE)
+#if defined(__clang__) && defined(__apple_build_version__)
+#if __apple_build_version__ <= 8000042
+#define JSONCPP_TEMPLATE_DELETE
+#endif
+#elif defined(__clang__)
+#if __clang_major__ == 3 && __clang_minor__ <= 8
+#define JSONCPP_TEMPLATE_DELETE
+#endif
+#endif
+#if !defined(JSONCPP_TEMPLATE_DELETE)
+#define JSONCPP_TEMPLATE_DELETE = delete
+#endif
+#endif
+
 #include <array>
 #include <exception>
 #include <map>
@@ -390,8 +408,8 @@ public:
   bool isObject() const;
 
   /// The `as<T>` and `is<T>` member function templates and specializations.
-  template <typename T> T as() const = delete;
-  template <typename T> bool is() const = delete;
+  template <typename T> T as() const JSONCPP_TEMPLATE_DELETE;
+  template <typename T> bool is() const JSONCPP_TEMPLATE_DELETE;
 
   bool isConvertibleTo(ValueType other) const;